Environmental Water Requirements in Mountainous Areas

Environmental Water Requirements in Mountainous Areas presents comprehensive and scientifically sound approaches and methodologies for estimating the environmental water requirements and tradeoffs for water allocation by analyzing anthropogenic and natural water needs. The book covers environmental water management issues in mountainous areas, specifically focusing on the Mediterranean region which exhibits significant contrasts in its demographic and hydrologic features. The authors include paradigms and information that will be useful for water resources managers, decision makers, scientists working in the fields of ecology and water resources management, engineers that design hydraulic works, and environmental policymakers.

• Offers a complete background screening on theoretical and practical guidelines on estimating environmental water requirements in mountainous areas
• Promotes and guides interdisciplinary work with information on policies and best practices in the field of ecological flows and water resources management
• Provides examples and case studies on the successful implementation efforts of ecological flows to analyze lessons learned and overcome practical issues and solutions

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 19, 2021
• ISBN: 9780128193433

Book preview

Chapter 1

Mountainous areas and river systems

Nikolaos Th. Skoulikidis,    Department of Inland Waters, Institute of Marine Biological Resources and Inland Waters, Hellenic Centre for Marine Research, Athens, Greece

Abstract

Mountains and mountain rivers provide a multitude of invaluable goods and services to a profound portion of the planet’s population. As water towers of the Earth mountains are sources of the mightiest world rivers and play a pivotal role for global biodiversity, freshwater, and sediment supply. Distinct morphological, climatic, hydrological, hydrochemical, and biological features of mountainous river ecosystems, compared to lowland ones, make them particularly fragile and vulnerable to human interference. Despite a number of remote mountain areas and rivers still remaining intact from direct human pressures, the majority of mountain ecosystems, are being increasingly threatened by adverse local and global changes driven by market economy. To efficiently conserve and sustainably use mountain ecosystems and contribute to the survival of the planet, it is critical to change our standards and life attitudes by realizing and appreciating our immediate connection to the global ecosystem, change attitudes and current consumption patterns, and stimulate the ways our global society functions and interacts with the natural environment.

Keywords

Mountains; montane landscapes; mountain rivers; river geomorphology; river hydrology; river chemistry; river biodiversity; sediment transport; human pressures; global climate change

1.1 Introduction

Mountains are landforms that extend above the surrounding landscape and are formed where the earth’s surface has been pushed upward through tectonic forces. As drivers of climate and vegetation patterns, storehouses of clean water, areas of rich biodiversity, and substantial contributors to global plant and animal production, mountains control human wellbeing in general. Water for domestic use, irrigation and hydropower (HP), flood control, mineral resources, timber, and medicinal plants are key mountain resources and services, while traditional agricultural practices promote sustainable production systems.

As a result of steep relief, tectonic activity and intense erosion under the forces of wind and water, mountains provide a multitude of scenic fluvial landforms ranging from quiet springs and brooks and ephemeral and glacier streams to steep whitewater torrents, bedrock streams cascading through spectacular gorges with multistep waterfalls, meandering, and braided channels. Mountains substantially contribute to the global freshwater runoff and are the sources of the Earth’s mightiest rivers. Certain physical processes occurring on high elevation create high surface runoff rates and mountain rivers supply large amounts of clean water and fertile sediment to the lowlands serving agriculture, irrigation, domestic, and HP uses.

Because of growing populations and needs for economic development, mountain areas, and mountain waters are increasingly subject to a manifold of conflicting interests and multipressures, exacerbated by accelerating climatic change impacts. As a result, mountain areas are losing their naturalness and wilderness, while water resources, sediment fluxes, and biodiversity decline, affecting their invaluable services they provide to the humanity. It is thus in our immediate interest to direct our efforts toward the effective protection of mountains and mountain rivers.

This chapter summarizes the characteristics of the world mountains and their ecosystems, presents the physiography, biodiversity, and processes that characterize the rivers, of montane regions, and outlines how natural characteristics of mountain areas and rivers are modified by human pressures.

1.2 Mountain areas

1.2.1 Defining a mountain

A common definition for a mountain area, adopted by UNEP-WCMC (2002), is a lower limit of 300 m. Kapos et al. (2000) used criteria based on altitude and slope to define six elevation classes and estimated the global mountain area by almost 40 Mkm², or 27% of earth’s surface. Meybeck et al. (2001) differentiated mountains from hills by their higher mean elevation (>500 m) and from plateaus by their greater roughness, that is, >2% at low and medium altitudes (500–2000 m) and >4% at high (2000–4000 m) and very high altitudes (4000–6000 m), and estimated 33.3 Mkm² or 25% of the earth’s surface to be mountainous. Viviroli et al. (2007) defined as mountains elevated areas that potentially deliver disproportional high runoff. They extended the definition of Meybeck et al. (2001) resulting in a 39% of global continental surface represented by mountains and higher-altitude areas (i.e., hills, mid-altitude plains, and platforms of medium to very high altitude). According to Rodríguez-Rodríguez et al. (2011), mountains cover 26.5% (39.3 Mkm²) of the world’s total terrestrial area, or 24.8% (33.3 Mkm²) of the terrestrial area outside Antarctica. More recently, Körner et al. (2016) combined geographical information with bioclimatic criteria (temperature), and using seven thermal life zones, arrived at 16.5 Mkm² or 12.3% of all terrestrial land area outside Antarctica being mountainous. According to Bridges (1990), mountain regions cover 52% of Asia, 36% of North America, 25% of Europe, 22% of South America, 17% of Australia, and 3% of Africa, as well as substantial areas of islands, including Japan, New Guinea, and New Zealand.

1.3 Mountain ecosystems

Most mountains and mountain ranges belong to two main mountain belts that have formed where lithospheric plates have converged with another, and in most cases continue to do. A nearly continuous chain of volcanoes and subduction mountain ranges surrounds most of the Pacific basin, the so-called Circum-Pacific System. On the east they form the spine of New Guinea, Japan, the Philippines, and Kamchatka Peninsula and to the north the Aleutian volcanic islands chain. Along the western edge of the Pacific basin emerge the mighty North American and Andean Cordilleras. Another major mountain range stretches the length of the Indonesian Archipelago. The second nearly continuous chain of collision mountain ranges, the Alpine-Himalayan (or Tethyan) System, can be traced from Morocco in North Africa through Europe, then across Turkey and Iran through the Himalayas to Southeast Asia (Fig. 1.1). Nearly all mountain ranges on the Earth can be included in one of these two major systems. The largest and highest area of mountain lands occurs in the Himalaya-Tibet region. The Himalayas comprise the most spectacular collision mountain range, with Mt. Everest peaking up to 8848 m. There are in total 14 mountains over 8000 m, all located in the two highest mountain ranges in the world, the Himalayas and the Karakoram in Nepal, Pakistan, and China. The longest nearly continuous mountain range is located along the west coast of America, from Alaska in the north to Chile in the south. Other significant areas of mountain terrains include those in Europe (Alps, Pyrenees), Asia (Caucasus, Urals), New Guinea, New Zealand, and East Africa.

Figure 1.1 World’s main mountain ranges and selected major rivers per continent.

Mountain meteorology typically includes greater spatial variability than adjacent lowlands because of complex interactions among global, mesoscale, and local atmospheric forcing (Barry, 2008). The most distinct climatic characteristics distinguishing mountains from surrounding lowlands are: decreased atmospheric pressure, density, temperature and humidity, reduced oxygen availability and dust content, increased insolation and enhanced receipt of solar radiation. Precipitation also tends to increase with altitude, particularly for temperate latitudes (Wehren et al., 2010). The complex topography in mountainous regions affects the distribution of precipitation and the retention of ice and snow. Precipitation height and type is strongly influenced by orography, aspect, and elevation, and show high intramountain variability. As air masses rise to pass over the mountains, the drop of air temperature causes water vapor to condense and fall as precipitation. Precipitation is heavier on the windward side of a mountain barrier (orographic precipitation) than on the leeward where many mountain ranges exhibit a pronounced rain shadow. For example, there is a sharp difference between the well-watered by the monsoons south facing slopes of the Himalayas in India, Pakistan, and Bangladesh and the dry deserts of China in the rain shadow of the Himalayan’s north-facing slopes. Death Valley, a desert in California and Nevada, is so hot and dry because it is in the rain shadow of the Sierra Nevada mountain range. Another profound example of orographic climatic features is the Chinook. Chinook winds develop when warm, moist air blows from the Pacific Ocean toward the Rocky Mountain range and brings rain or snow to the peaks. After releasing its moisture in the mountains, the air mass dries and warms as it moves down the eastern side of the mountains. In high mountainous regions, mountain glaciers form, often flowing out of ice fields spanning several mountain peaks or mountain ranges. Mountain orography gives rise to many local winds, chief among which are the foehn, mountain and valley winds, mountain-gap winds, and downslope winds. On many tropical mountains, the forest zone extends into the level of average cloud height, which causes an excessively damp climate and produces the so-called fog forests.

From an ecological point of view, mountains may be classified in the following categories (Grabherr & Messerli, 2011): Arctic mountains, among which Greenland is outstanding in terms of size and latitudinal expansion, predominately covered by vast ice sheets. They are located north of the arctic tree line and are among the most unfriendly environments for humans, and therefore remain in an almost pristine state. Boreal mountains extending north of 50-degree latitude cover approximately 5% of the Earth’s terrestrial surface. Examples include the Canadian Rockies, the Caledonian mountains of northern Europe, the northern and polar Ural, and Siberian ranges. Boreal mountains are covered by large coniferous forests (taiga), except of oceanic mountains where deciduous forests or shrublands dominate, and as hostile environments are only scarcely inhabited. Temperate mountains belong to middle latitudes with strong climatic seasonality and frosty winters. Depending on precipitation, vegetation ranges from deciduous forests to steppes and prairies, or even deserts, for example, in continental Eurasia and Northern America, where mountains reach up far into the glaciated nival zone (Cascades, Rocky Mountains, Alps, Caucasus, Tienshan in Central Asia, Southern Altai in west Siberia). The Himalayas and the highest peaks of Tienshan are outstanding examples of the above 6000 m eolian zone environments with very limited life occurrence. Temperate mountains of the Southern Hemisphere include the New Zealand Alps and the Andes of Chile and Argentina between 35 and 45 degree latitude. Subtropical mountains include those belonging to the Mediterranean and hot desert life zones. Here belong the mountains surrounding the Mediterranean Sea, the Californian Sierra Nevada, the Snowy Mountains in Australia, and the Andes of central Chile between 33 and 35 degrees latitude. Evergreen sclerophyllous forests and shrublands dominate, replaced by deciduous forests in higher altitudes. Between 22 degrees and 25 degrees N, the Andes, and between 20 degrees and 24 degrees N the mountains of the central Sahara rise up from hot deserts. There, even at higher elevations, precipitation is too low to support forests growth. In tropical mountains, climate ranges from seasonal (Ruwenzoris, Sierra de Santa Martha, Mount Willem, and Kinabalu) to slightly seasonal (e.g., Peruvian Andes, Kilimanjaro). Above 4000 m frost occurs in the night and the highest summits are glaciated. The predominant vegetation types along the elevational gradient include montane rain forests and cloud forests. Mountains with alternating rainy and dry season (the Andes between 14degrees and 30 degrees S, and mountains on islands, e.g., Hawaii), remaining dry at the lowlands, are characteristic for the area close to the tropics.

1.4 Mountain resources and services

Mountains are storehouses of water and global biodiversity and account for 32% of the Earth’s surface runoff (Meybeck et al., 2001). Due to the extreme heterogeneity of environments, rapid elevational changes, and variable directional orientation, mountains have diverse vegetation and varied microclimatic and ecological conditions (Sharma et al., 2010). As a consequence, they are rich in endemic species, supporting approximately one-third of terrestrial biodiversity (Körner & Paulsen, 2004), and host half of all 34 global biodiversity hotspots (Chape et al., 2008). Mountains play a major role in influencing the climate and vegetation patterns, and are also important centers for tourism and renewal. Mountains encompass some of the most spectacular landscapes and harbor a significant portion of distinct ethnic groups, varied remnants of cultural traditions, environmental knowledge, and habitat adaptations (Spehn et al., 2010).

Mountains directly support 13% of the world’s population.¹ They deliver half of the Earth’s population with good water quality, and mountain agriculture provides subsistence for about half a billion people (Körner & Oshawa, 2006). Freshwater, usually clean and safe for human consumption in its natural state, is considered to be the most important product of mountains for economic wellbeing. As storehouses of water, mountain ranges supply water for drinking, domestic, irrigation², and industrial uses and play a key role in providing renewable energy, especially through HP, wind, and solar power for downstream cities and remote mountain communities. Forests are probably the second most important good provided by mountains. Timber is a mountain resource that is readily converted to a marketable commodity through logging. Standing timber also provides valuable services by stabilizing water flow, and protecting biodiversity. Minerals are another major good provided by mountains. Worldwide, mountain herbs are traditionally used in folk medicine, including the treatment of diseases as, for example, Tibetan doctors use to practice (Salick et al., 2006). Finally, biodiversity benefits are among the most widespread of mountain values, but among the most difficult to assign market prices (Pratt & Shilling, 2003).

As the highest and most impressive features of the landscape, mountains have an unusual power to awaken a sense of spirituality. The Incas were the first who climbed at an elevation of 6700 m to perform religious rituals at c. 1500 BP³. Sacred mountains were/are central to certain civilizations, such as Mount Olympus in ancient Greece, currently a National Park, and the ten sacred Mongolian mountains protected by Presidential Decree in our modern times. Mountains encompass some of the most spectacular landscapes and harbor a significant portion of distinct ethnic groups, varied remnants of cultural traditions, environmental knowledge, and habitat adaptations (Spehn et al., 2010).

Healthy mountain landscapes are fundamental for providing mountain services, the latter being essential for facing major challenges of the 21st century, such as clean and reliable water supplies and food security. However, the need to protect mountains evolves not only from the multitude of essential services that provide but also from mere wilderness and uniqueness conservation characteristics and from preservation of cultural landscape aspects. The notion of high cultural diversity (e.g., ethnic diversity, minorities, and endangered languages) and the idea of mountains as very special places (Hamilton & McMillan, 2004) charged with spiritual values additionally support mountain conservation efforts (Kollmair et al., 2005).

1.5 Mountainous rivers and streams

Mountains are the sources of the planet’s mightiest rivers; the Amazon with its major tributaries (Madeira, Rio Negro, and Japura) and the Parana rise on the windward eastern slopes of the Andes and traverse the South American continent to reach the Atlantic Ocean (Fig. 1.1). The Orinoco springs at the southern end of the Parima Mountains. The Missouri and Mississippi in North America rise on the eastern slopes of the Rocky Mountains. The water towers of Asia, the great Indus, Ganges, Brahmaputra, Mekong, Yangtze, and Yellow rivers flow from the windward southern slopes of the Himalayas and the Tibetan plateau, the Lena springs in the Baikal Mountains south of the Central Siberian Plateau, and the Danube, Rhine, Rhone, and Po rivers rise on the northern slopes of the Alps.

Mountainous rivers and streams encompass a great variety of landscape diversity. Many of them illustrate spectacular scenic esthetic beauty and are recognized as centers for relaxation and inspiration, thus attracting nature friends, artists, and philosophers. The ancient Greek philosopher Heraclitus of Ephesus (c. 535–c. 475 BCE) was inspired by the image of a flowing river to express his famous philosophical aphorisms: πάντα ῥεῖ (panta rhei), that is, everything flows, everything changes, and No man ever steps in the same river twice or We both step and do not step in the same river, We are and we are not. Since the times of the Han Dynasty (206 BCE–CE 220), painters in China expressed their philosophical communion to nature with depictions of mountains and rivers.

A number of remarkable mountain rivers are parts of the UNESCO Global Geoparks⁴, among them the Stonehammer in Canada, located at the confluence of the Saint John and Kennebecasis rivers, the Non Nuoc Cao Bang in Viet Nam, that includes a complex drainage with five major river systems, the Yuntaishan in China with impressive multistep waterfalls, the spectacular Vikos-Aoos and Vouraikos mountainous river gorges in Greece, etc. Many mountain rivers are also included in ecosystems designated as UNESCO Biosphere Reserves and of course there are countless mountain rivers of extraordinary beauty around the world not officially recognized.

Ancient cultures, such as the ancient Greeks, the Hindus, the Maori, the native Americans, and among others, recognized the vital role of rivers in life and worshiped them as gods. The Ganges and other six Indian rivers, the Urubama of Peru, the Jordan in Israel, and the Osun in Nigeria are still considered sacred. The Whanganui in New Zealand became the first river in the world to be granted the same legal rights as a human being. However, rivers around the world are affected by multiple human pressures. Although mountain rivers are much less endangered and many of them remain in nearly undisturbed conditions, increasing population and pressure for economic growth is progressively threatening them through urbanization, mining, clear cutting, and renewable energy production, thus amplifying the effects of climate change.

1.5.1 Hydrogeomorphological characteristics

In contrast to self-formed flood-plain channels, the gradient and morphology of mountain channels are tremendously variable and prone to forcing by external influences (Montgomery & Buffington, 1997). The major mountain belts of our planet are still forming today, all along boundaries of active tectonic plates. Tectonic rise is matched by the forces of erosion, and thus headwater streams⁵ are dominantly erosional. Mountainous channels show sharp differences from lowland ones in valley side slopes, relief, and relief ratios and are likely to have strongly segmented longitudinal profiles with pronounced and abrupt variations in gradient, valley width, channel pattern, and grain size, related to substrate resistance and climatic and tectonic history (Wohl, 2010).

In mountainous zones, catchments are mostly confined to elongated valleys and may develop palmate and pinnate dendritic drainage channel patterns⁶ (Naiman et al., 2005). Fold mountains tend to have trellis drainage patterns⁷, while radial drainage patterns⁸, often develop on topographic domes, such as volcanic cones (Huggett, 2007). In steep mountainous terrain, streams flow rapidly through high gradient narrow, deep valleys with steep banks over rocky stream bed. Stream forces erode their bedrock along their courses and carve complex patterns into the landscape, cutting deep canyons. Streams are marked by narrow, V-shaped valleys and channels that fill most of the valley floor. Valley floor profiles commonly show an asymmetry on opposite sides of the valley caused by differences in weathering regime (Wohl, 2010). Steep long profiles are broken by waterfalls, rocky rapids, pebble, and log steps, which disperse energy (Bravard & Petit, 2009). Mountain channels typically have a gradient ≥0.2% along the majority of their length (Jarrett, 1992), and as water and sediment move quickly, they accomplish highly dynamic hydrogeomorphological processes. Though truly straight channels are quite rare (Leopold & Wolman, 1957), straight reaches may develop down into solid rock in steep mountainous areas through water flowing rapidly downhill. Alluvial fans are another characteristic feature of mountainous drainages in which channels exit laterally confined canyons and enter a glacial trough or other wider valley segment, an intermontane basin, or a piedmont environment (Wohl, 2010).

Wohl (2010) lists several typical characteristics of mountain rivers, such as: high slope, erosion resistance, and hydraulically rough channel boundaries associated with bedrock and coarse clasts, highly turbulent flow with large variations between critical and supercritical flow⁹, limited supply of fine sediments, bedload movement, strong seasonal and spatial discharge and bedload regime, large longitudinal variation in channel geometry, and relatively narrow valley bottoms.

Build upon Schumm’s (1977) concept of erosion, transport, and deposition zones, Montgomery and Buffington (1997) classified mountain rivers into source, transport, and response reaches and recognized three respective valley segment and channel-reach substrate types: colluvial, alluvial, and bedrock. Stream valleys can be filled with colluvium, which is usually immobile except during extreme hydrologic events, or alluvium, which may be frequently moved by streams, whereas bedrock valleys are dominated by bedrock outcrops.

Colluvial valleys serve as temporary repositories for sediment and organic matter eroded from surrounding hillslopes. Colluvial channels are small headwater streams that flow over a colluvial valley fill and exhibit weak or ephemeral fluvial transport ineffective at removing materials deposited on the valley floor. Consequently in steep headwater channels, episodic transport by debris flows may account for most of the sediment transport. The accumulation of colluvial valley fills during periods between catastrophic scouring events indicates that transport capacity, rather than sediment supply, limits fluvial conveyance in colluvial channels (Dietrich et al., 1986; Montgomery & Buffington, 1997; Bisson et al., 2006).

Alluvial valleys are characterized by a temporally and spatially continuous blanket of transportable sediment on both the bed and banks and exhibit a wide variety of morphologies, that is, cascade, step-pool, plane-bed, pool-riffle, and dune-ripple. As going from sharp to more smooth morphologies, reaches are becoming less steep, less constrained and wider, and evolve from sediment transport to depositional zones.

Bedrock rivers dominate in areas of net erosion and encompass most mountain rivers (Wohl & Merritt, 2008). Bedrock channels occur mainly, but not exclusively, in actively incising portions of landscapes and where channels are cut into resistant rock units, most often in actively uplifting areas (Whipple, 2004). Shear stress¹⁰ on hillslopes builds up over time as slopes become steeper, and rivers drive this steepening through downcutting, or valley incision. Downcutting is most rapid in mountainous areas where river gradients are steepest (Naiman et al., 2005). Bedrock valleys are steep, typically confined by valley walls, and lack contiguous alluvium cover as a result of high transport capacity relative to sediment supply. Thus the bed and banks of bedrock reaches may be composed of in place bedrock.

Focusing on smaller scales, channel reaches consist of repeating sequences of specific types of channel units. The most generally used channel unit terms for small to midsize streams are riffles and pools. Channel units may be classified into falls, fast water units, smooth fast water units, scour pools, and dammed pools. Among the fast water channel units belong falls, cascade, chute, rapids, and riffles (Hawkins et al., 1993).

However, river habitat is not static as the distribution of the habitat mosaics changes spatially over time due to primary drivers, particularly flooding, channel avulsion, cut and fill alluviation, deposition of wood recruitment and regeneration of riparian vegetation (Stanford et al., 2005), forcing functional and structural features of biological communities to adjust to these changes (Vannote et al., 1980). Although mountain rivers usually have relatively narrow valleys with poorly developed floodplains, wider valley segments can result from glaciation, beaver activity, or reduced erosional resistance of the local bedrock, and these valley segments can have well-developed floodplains (Wohl, 2010). Hillslopes lateral floodplain extent and its variability within the drainage basin exert a particularly important control on stream water groundwater interaction within the floodplains, affects hydrological features and substantially impacts stream water chemistry and temperature. Since riverine floodplains are one of the most endangered landscapes worldwide (Tockner & Stanford, 2002), undisturbed mountain streams are of profound importance in maintaining floodplain functions.

1.6 Hydrological characteristics

Almost all major rivers and many of the world’s minor rivers originate in mountains (Pratt & Shilling, 2003). Estimates on the contribution of mountains to the global freshwater runoff vary between 32% (Meybeck et al., 2001) and 40%–60% (Bandyopadhyay et al., 1997). According to Körner and Oshawa (2006), mountains contribute 20%–50% of the total discharge in humid areas, and 50%–90% in semiarid and arid areas. In a catchment scale, mountain flow may reach even 95% of total flow (Liniger et al., 1998). Mountains in the arid zone clearly deliver most disproportional discharge (66.5%) compared to their share in total area (29.8%). Temperate zone mountains show the second highest disproportionality (share in total area: 43.4%, share in total discharge: 60.8%) (Viviroli et al., 2007). For example, the European Alps cover only 23% of the Rhine River basin, but provide half of the total runoff over the year (Marston & Marston, 2017).

Orographic rainfalls, high rates of precipitation, long retention of precipitation in the form of ice and snow, snow melting, reduced evapotranspiration and infiltration, vapor condensation, and restricted groundwater aquifers, contribute to considerable higher mountain specific discharge than in lowland areas (Dingman, 1981). Runoff coefficients rise dramatically as soil cover declines above 1000 m above sea level (asl) and areas with regolith cover increase (Wehren et al., 2010). Some mountains have permanent coverings of snow and ice, with the Himalayas being the largest storehouse outside the polar region (Vohra, 1993), providing over 50% of the Indus River flow with melt water (Bookhagen & Burbank, 2010). Water seasonally stored in snow and ice contributes to the runoff of lowlands during the spring and summer melt period when the water demand is higher. Cloud and fog condensation can also contribute to stream runoff. Abundant fog deposition occurs in high-altitude forests, on ridges and peaks. Locally, vapor condensation may account for a significant portion of total precipitation (Grunow & Tollner, 1969; Turner, 1985; de Jong, 2005) and is probably the most common source of water at altitude zones between 2000 and 3000 m asl (Wehren et al., 2010).

Stream channel initiation is a key factor in the evolution of mountain landforms. Channel initiation happens when surface or subsurface flow is sufficiently concentrated and persistent to produce a discrete channel. The location of the channel-head¹¹ is affected by multiple drivers, such as gradient, drainage area, infiltration, and permeability/porosity, that control overland, subsurface, and debris-flow (Wohl, 2010) and determine the pathways by which hillslope runoff will reach the stream channel (Burt, 1996). More than 95% of the water flow passes over or through a hillside and its regolith before reaching the channel network (Kirkby, 1988). Vegetation can also concentrate the movement of precipitation toward the ground surface via stemflow, or water flowing down the trunk or primary stems of a plant, thus initiating rills (Wohl, 2010). Below the channel head and above the stream head (that is characterized by perennial flow), channel segments of intermittent and ephemeral flow are commonly present.

Headwater streams account for most of the drainage network of mountain rivers and supply water, sediment, and wood to downstream reaches (Gomi et al., 2002). The processes by which water moves downslope exert an important control on the hydrology and chemistry of headwater streams and mountain rivers. Precipitation falling toward a hillslope can be intercepted by plants and evaporate or transpire back into the atmosphere. The remaining water is either flowing surficial on the ground or is infiltrated in the subsurface. Infiltration depends on precipitation intensity and duration and precursor moisture, as well as surface porosity and permeability, soil hydraulic conductivity, and water retention. In turn, surface porosity and permeability are controlled by hillslope gradient, vegetation, regolith grain size, compaction, depth, and areal extent (Wohl, 2010). When soils are thin or absent, as in desert mountains, infiltration rates are low (Marston & Marston, 2017). If the infiltration capacity of soils, regolith or rocks is low relative to precipitation intensity, precipitation reaching the ground can flow downslope at the surface as Hortonian overland flow¹² (or infiltration excess overland flow). In mountain areas with thick regolith and forested mountain soils, infiltration rates are commonly very high, and subsurface flow dominates, because of the coarse soil texture (Dunne & Black, 1970). Infiltrating water that enters the subsurface can move downslope in the soil-water zone as throughflow, in the unsaturated zone below plant roots as interflow, or below the water table as groundwater flow. Beneath streams, water moves laterally as intergravel flow. When soil pores are filled to capacity and become completely saturated, saturation-excess overland flow may occur (Dunne, 1978). This flow component consists of direct precipitation onto saturated areas and return flow from the subsurface as saturation occurs. Thus saturation-excess overland flow is a mixture of direct runoff (water with generally low solutes, unable to infiltrate into saturated soil) and old water (rich in solutes) stored in the ground (Burt, 1996).

In terms of flow regime, mountain streams can be classified as perennial, intermittent, ephemeral, or glacier melt, snowmelt, rainfall, rain-on-snow, combined runoff sources, or other subdivisions (Hannah et al., 2005). In ice-free montane basins, maximum monthly discharge occurs in spring, annual runoff is less than annual precipitation, and intraannual runoff variations follow those of precipitation (Collins, 2006). The annual stream flow hydrograph in snow and glacier-fed rivers is characterized by low flows during winter when water is stored in the seasonal snowpack and glaciers, and high flows during spring and summer when snow and glaciers melt (Déry et al., 2009). In glacierized basins, the greater the ice cover, the later into summer occurs maximum monthly discharge, the higher the annual runoff, which may exceed annual precipitation, and the more runoff variability is influenced by intraannual changes in thermal energy available for melting (Collins, 2006). As a result, seasonal discharge patterns of mountainous flow substantially smooth seasonal and long-term discharge variation at the lowlands (Körner & Oshawa, 2006).

Since mountains are situated in a wide range of climate regions, they illustrate profound differences in flow regime (Wohl, 2010). Marston and Marston (2017) distinguished eight regional types of flow regime: Mountains in warm-arid climates (e.g., Basin and Range Province of the United States, Middle East, western South America, Africa, and Australia) illustrate ephemeral flows with occasional flash floods. Cold-arid mountains (e.g., parts of Antarctica) experience ephemeral flows during the melt season. Semiarid mountains (e.g., the Colorado Front Range of the United States, the Pyrenees of Europe) can experience ephemeral flow from rainfall or perennial flow from snowmelt. Mountains in warm-humid climates (e.g., Appalachian Mountains, mountains of Japan) can generate perennial flows, occasionally with very high discharges caused by tropical storms. Mountains in cold-humid climates (e.g., the western side of Canadian Rocky Mountains, the European Alps, and the Chilean Andes) also support perennial flows, dominated by either rainfall or snowmelt. Mountains of the seasonal tropics (e.g., the Northern Great Dividing Range of Australia, the Caribbean region) experience strong seasonal shifts in discharge, in some cases highlighted by monsoon seasonal flooding. Mountains in the humid tropics (e.g., in New Guinea, the headwaters of the Amazon) tend to be perennial, with lower seasonal variation than in other types of mountain rivers. Finally, high-elevation mountain massifs are dominated by snowmelt and glacier melt, although the Himalayas are additionally dominated by monsoon rains.

Because of the close coupling between hillslopes and channels, floods along mountain rivers differ from those in the lowlands (Wohl, 2010). Hydraulics of high gradient streams are strongly affected by large boulders and woody debris which create bed roughness leading to highly turbulent flow, and rapid flow (Grant, 1989). One of the effects of riparian forest expansion over the last century in Europe was the intensification of large wood recruitment and increased flood hazard resulting from wood transport and deposition during flood events (Stoffel et al., 2016). Floods in mountain rivers are favored by the typically steep channel gradients and can be generated by various types of rainfall, rain-on-snow, and snowmelt (Weingartner et al., 2003). In high-latitude mountain regions, ice jams can also exacerbate flooding during the spring (Buzin, 2000). Large/extreme flood events are caused by high-intensity convective rainfall, extensive deep cyclones generating a few days-long, orographic rainfall, and outburst floods resulting from the failure of either natural or artificial dams (Stoffel et al., 2016). Extreme floods can be disastrous, resulting in substantial material losses and large numbers of fatalities, if they affect managed valley sections and inhabited valley