Field Guide to California Rivers

By Tim Palmer

Award-winning author, naturalist, and conservationist Tim Palmer presents the world of California rivers in this practical and inspiring field guide. Loaded with tips on where to hike, fish, canoe, kayak, and raft, it offers an interpretive approach that reveals geology, plant and wild life, hydrologic processes, and other natural phenomena. Palmer reports on conservation with a perspective from decades of personal engagement. More than 150 streams are featured, 50 riparian species are illustrated, and 180 photos show the essence of California’s rivers. Palmer brings a natural history guide, a recreation guide, and an introduction to river ecology together in one illuminating volume; it belongs in every river lover’s book collection, boat, and backpack.

• Language: English
• Publisher: University of California Press
• Release date: Apr 30, 2012
• ISBN: 9780520952195

Tim Palmer

Tim Palmer is an author and photographer of environmental issues, river conservation, nature, and adventure travel. His thirty-two books have won numerous awards. For the past five decades he has been professionally and personally involved in flooding and issues of floodplain management. See his work at www.timpalmer.org.

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AN INTRODUCTION TO THE RIVERS

THE NETWORK OF RIVERS

Most California rivers flow into the Pacific, but some flow into landlocked basins of the interior deserts. This field guide groups the rivers into six hydrologic regions. Each includes a specific basin (also called a watershed, which is all the land that drains into a river), or a collection of smaller but similar basins, such as the North Coast region.

Most water flows in just four rivers: the Sacramento, Klamath, Eel, and San Joaquin. The combined North Coast rivers along with the Sacramento and San Joaquin carry 90 percent of the state’s total runoff. Most people and most farmlands lie south of Sacramento, and so the development of California’s rivers has been a story of tapping these northern and Sierra Nevada rivers and diverting them south or to the coast.

California’s Largest and Longest Rivers

The state’s largest river in volume of flow is the Sacramento, followed by the Klamath and Colorado, though by the time the Colorado leaves the state it’s diverted with little left in the channel. The longest river under one name is the 374-mile Sacramento, though substantial mileage is tidal in the delta and San Francisco Bay. The Pit, however, is the upper Sacramento’s largest source, and when combined with the Sacramento River below its mouth, totals 540 miles. Similarly, though small in volume, the San Joaquin with its South Fork source totals 393 miles—longer than the Sacramento or Klamath. The table that follows lists the volume of the largest rivers plus the mileage of major rivers plus their key headwaters tributary—in effect the same stream under a separate name. For data methods, see the beginning of the river profiles.

North Coast Rivers

The North Coast region is soaked by winter rains, and rivers here account for 43 percent of California’s runoff. They wind through conifer-clad mountains and green valleys that are fog-shrouded or sun-basked depending on the month of the year, and they carry powerful winter flows to sea-level estuaries or ocean beaches at the dramatic coastal escarpment that continues almost uninterrupted from San Francisco to Oregon. The Klamath is the largest waterway here, beginning in the interior mountains of Oregon and cutting an epic course across the southern Cascade, Klamath, and Coast ranges.

Ukonom Creek—a Klamath tributary—illustrates the lush forests, clear water, and undeveloped watersheds of the North Coast.

The other major river of the north is the Eel, with tributaries including the Van Duzen, Middle Fork, and South Fork. From the Smith, at the Oregon border, through the Eel, the rivers are relatively dam-free and clean, with much of their watershed acreages in public ownership, with surviving though imperiled runs of salmon and Steelhead, and with far less development pressure than elsewhere. Though not without problems, this watery hinterland of northern California is one of the most outstanding regions of wild rivers in America. South of the Eel, the rivers are relatively small, except for the Russian as it flows through an interior valley toward San Francisco Bay but then turns sharply out to sea.

Sacramento Basin

Draining the interior of northern California, this is the state’s largest river, carrying about 31 percent of total runoff. Steep headwaters foam through shading forests until trapped behind Shasta Dam, followed by a long passage through the northern half of the Central Valley, where most of the frontage is confined by levees and farmland.

The Pit—a tributary that’s larger in volume and longer than the Sacramento where they meet—collects hearty spring discharges of the Fall River along with Hat and Burney creeks. This remote northeastern corner of California is the fly-fisherman’s paradise, with spring creeks that rival the legendary streams of Yellowstone. Farther south, a series of fairly intact creeks aim west from Lassen Peak at the southern limits of the Cascade Mountains.

A Sacramento tributary, the South Fork American is California’s most popular paddling stream.

A small set of Sacramento tributaries drains the eastern slope of the coastal mountains, but most streams there sink into gravel or are depleted for irrigation before reaching the river. In the lower basin, Cache Creek is the largest Sacramento tributary from the west and carves wild canyons.

The third set of tributaries drains the Sierra Nevada and provide by far the greatest flows to the Sacramento, beginning with the Feather River and continuing through the American. The lower Sacramento eases past the state capital nearly at sea level, and just below, the Sacramento Delta is one of the largest deltas in the nation.

San Joaquin Basin

California’s third hydrologic region, the San Joaquin River Basin covers the southern half of the Sierra and occupies the southern Central Valley. Though it’s larger than the Sacramento Basin in area, precipitation decreases in the south, so this river carries only 11 percent of the state’s runoff—one-third the Sacramento’s volume—and most is diverted by irrigators. Nearly all the water comes from Sierra Nevada tributaries extending from the Cosumnes south through the high-mountain headwaters of the San Joaquin.

The Tuolumne River typifies the granite canyons of higher elevations in the San Joaquin Basin.

South of the San Joaquin, the added volume of the Tulare Lake Basin, which historically overflowed into the San Joaquin, would raise the greater San Joaquin’s total to about 16 percent of California runoff. Flowing toward the dried-up Tulare Lake, the Kings River churns from alpine terrain through its Middle and South forks in Kings Canyon National Park. The Kaweah, Tule, and Kern likewise flow from high mountains to the southern Central Valley.

All the major San Joaquin tributaries are dammed except the diminutive Cosumnes and Clavey, along with some forks of the larger streams. Hydropower dams were built even at high elevations. Yet impressive free-flowing mileage remains. Stellar reaches in the Sierra Nevada later tumble with rapids through the pines, oaks, and chaparral of the foothills, followed by languid windings through the Central Valley. These lower reaches are typically entrenched with wooded banks 10 to 20 feet high and farmland just beyond. Occasional parks are found, but most valley reaches are inaccessible to the public. The west side of the San Joaquin Valley—draining the interior Coast Range—sees little runoff because of a dry climate, the intervening Salinas River Valley to the west, and the rain-shadow effect of coastal mountains.

Central and South Coast

The state’s fourth hydrologic region is composed of the rivers at the Central and South coasts and accounts for 5 percent of statewide runoff. Streams flush quickly from the 500-mile-long Coast Range that rises south of San Francisco—longer than the Sierra but producing a small fraction of the water. Springtime—with streams flowing, riparian forests leafing, and hillsides green with grass and flowers—is the best time to see these small rivers.

The San Lorenzo is a major stream of the Central Coast as it bubbles down from redwood forests of the Santa Cruz Mountains. On the east side of the coastal mountains, the depleted Salinas riffles north to Monterey Bay. Along the South Coast, streams are even smaller. As they exit rugged canyons they drop bedloads of gravel in broad alluvial plains where remaining water seeps underground except during peak storms.

Eastern Sierra Nevada

Rivers of the Eastern Sierra account for only 4.5 percent of statewide runoff but are spectacular with steep gradients; this is unlike any other group of streams in America. The Truckee, Carson, and Walker fall from the Sierra escarpment. To the south, the Owens River picks up Sierra snowmelt from stunning tributaries and takes a remarkable path through the desert before being diverted to Los Angeles.

Deserts

In one of America’s driest regions, these streams provide only 0.3 percent of the California runoff, not including the giant Colorado River, whose volume comes entirely from other states. In the heart of the Mojave Desert, the Amargosa River ends in Death Valley. Palm Canyon Creek nourishes rare oases of fan palms. Marking the border of California and Arizona, the Colorado carries prodigious runoff from the Rocky Mountains but is completely tapped before reaching its estuary at the Gulf of California in Mexico.

All these rivers and streams are formed and governed by the great natural systems that surround them. Geologic processes and climate shape the hydrologic makeup of each stream and ultimately determine what plants and animals will thrive. Those natural processes and the life forms they support are the topics of the next section.

THE NATURAL HISTORY OF RIVERS

The fundamental nature of any river—the way it looks and works—is a result of five factors. First, geologic events created the mountain ranges, topography, and bedrock that a river flows through—the big backdrop. Second, climate governs the amount and timing of the all-important rain and snow. Third, the forces of geology and climate together determine the hydrology, or characteristics of flow, including cycles of floods and drought. This flow is what has sculpted valleys and canyons into the profiles we recognize today, and it continues to determine the morphology or shape of riverbeds as well as that of shorelines and floodplains. Fourth, the combined effects of geology, climate, and hydrology govern what plants can live in and along a river. Finally, plant life and all the other factors define what fish and wildlife will thrive. Fascinating relationships between flora and fauna affect the population and health of each species. The extraordinary physical and biological diversity of California rivers is due to the multitude of ways that all these interactions vary across the state.

The South Fork Mokelumne nourishes its green corridor of life near the Hwy. 26 bridge.

Geology

Plate tectonics is the process that initially created the landscapes we now know. Distinct sections of the earth’s crust float on a mantle and molten core, which allow the surface crust to move. In and along California, the Pacific Plate is slowly migrating north along the edge of the North American Plate, and the abrasive meeting place of the two is the San Andreas and related faults. The Pacific Plate boundary turns sharply out to sea at Cape Mendocino (not the town of Mendocino, but the westward-jutting headland north of the Mattole River), where the undersea Gorda Plate lies next to the North American Plate. Most of this heavier sea-bottom plate is pushed underground while the larger, lighter terrestrial plate glides westward over it. In the process, fragments of the down-bound crust break off at the subduction zone and accumulate as the Coast Ranges. As the subducted material is buried deeper and deeper, it heats, melts, and later erupts in volcanoes of the southern Cascade Range including Shasta and Lassen. A similar tectonic process formed the Sierra Nevada. Here a pluton of molten underground rock hardened into granite and then through faulting emerged to become the spectacular mountain chain we know, once overlying rock eroded away. Meanwhile, the Great Basin landscape east of the Sierra has been rifting or spreading apart to form parallel ranges that trap waters traveling between them. With this creation story of plate tectonics and mountain formation in mind, we’ll now consider some of the geologic forces specifically affecting rivers (for individual streams, see the river profiles).

In northwestern California, ancient plates or terranes long ago collided with the West Coast and pushed up today’s more easterly outliers of the Coast Range complex—for example, the Marble and Yolla Bolly ranges. Rivers forming in these mountains and progressing seaward became blocked by the rising of the newer and westerly coastal ranges and repeatedly found their paths of least resistance along the weakest rock strata following fault lines. These typically run northwest to southeast (parallel to the plate boundaries), and a number of streams flow on strikingly similar northwest routes to the ocean. This cant is evident at the lower Smith, Klamath, and Trinity; also in the northwest aim of Redwood Creek, Mad River, and the Mattole; and along much of the Eel and its South Fork (see map). The more southerly Russian and Salinas rivers parallel northwest–southeast fault lines in similar ways.

Pushed up by tectonic action where the Pacific and North American plates meet, and stretching north to south along much of the coastal edge of northern California, the Franciscan Complex is a large formation of mixed rock scraped off the bottom of ancient seafloors. Thoroughly fractured by seismic action, it was crumpled against the western edge of North America as part of the Coast Ranges. This broken, bent, and altered rock is highly susceptible to weathering, and is one reason that North Coast rivers have exceptionally erodible watersheds. The Eel, for example, carries the highest load of sediment as a percentage of water volume in the United States—even higher than the muddy Mississippi (one would not guess this from the Eel’s lucid green pools in summer; the silt is transported mostly during winter floods).

Farther inland, but still within the Coast and Klamath ranges, millions of years of seismic activity have mixed soft rock strata with outcrops of harder sandstone and granite. Where these resistant formations intersect with rivers, they create rapids. Gabbro and basalt are also hard rocks that resist erosion and result in gorges such as Oregon Hole on the Middle Fork Smith and Burnt Ranch on the Trinity. Throughout the greater coastal ranges, metamorphic and sedimentary rocks from previous ocean floors generally characterize the stream channels, unlike the northeast and the Sierra Nevada, where igneous rocks either monopolize or dominate.

In the northeastern reaches of the state, the Cascade Mountains were formed by volcanic eruptions. These included the great volcanoes, Shasta and Lassen, and also widespread lava flows at lower elevations. Once it hardens, lava is porous and allows water to accumulate in air spaces. These subterranean reservoirs ultimately discharge as groundwater and contribute to the network of spring-fed streams at the Modoc Plateau and Cascades. The Fall, Pit, McCloud, upper Sacramento, and Shasta rivers are all fed by spring discharges that emanate from hardened lava.

Lava serially seeped out of the earth in sheets, forming layers up to several hundred feet thick. This hard rock now resists erosion, and so streams in the volcanic regions drop over falls when they come to a fracture or to the edge of a lava formation. Burney Falls and the falls of the McCloud are spectacular examples.

All these northeastern groundwater-fed streams have cold currents due to the dark and chilled nature of their underground sources. They also have relatively steady flows, because the groundwater seeps slowly as spring discharges from natural reservoirs. The dual qualities of coldness and steadiness make these rivers prime trout habitat.

Though it’s not plentiful in the north or statewide, limestone is another ingredient that makes for biologically rich waters. Where it does occur, the carbonate breakdown products provide excellent building blocks for aquatic life. Even without limestone, many California streams are somewhat endowed with calcium, and with it, mussels and crustaceans readily form their shells and carapaces, and aquatic insects multiply, together forming the basis of aquatic food chains that support a host of fish, birds, and other wildlife.

South of the volcanic Cascades, the signature rock of the Sierra Nevada’s 400-mile-long range is granite. Boulders, cobbles, and gravel of gray and nearly white granite typify rivers from the North Fork of the Feather southward through the Kern. Unlike the porous volcanic rock of the Cascades, the Sierra granite sheds snowmelt rapidly in flushes of springtime and early-summer runoff followed by several months of low flow. In the high Sierra, rapids are often formed when rockslides of the resistant granite clog river channels. The magnificent waterfalls that characterize upper sections of many Sierra streams—especially from the Tuolumne southward—occur where water flows over massive slabs of granite. Other waterfalls occur where tributary streams from hanging valleys enter larger valleys scoured deep by glaciation. Ice filled these larger valleys to a height where the tributaries and their own glaciers flowed directly onto the great mass of trunk ice, and were unable to keep pace with the erosive rate, as side streams usually do.

Headwaters of Shadow Creek gather from snowmelt beneath the towering metamorphic monolith of Mount Ritter in the Middle Fork San Joaquin Basin.

One of the most consequential geologic factors affecting California rivers has been the presence of gold. In foothill elevations along the edge of the Sierra’s granite pluton, the northern Sierra has an eclectic mix of metamorphic, sedimentary, and volcanic rocks, including veins of quartz that were intruded from deeper plutonic sources and into the overlying mix of rocks. From these quartz intrusions, gold deposits broke free when the parent rock eroded, and eventually the gold fragments washed into rivers. Even tiny specks of gold are dense, and they settled on gravel bars and in low spots in the riverbeds. These water-delivered (placer) deposits extend from the Feather Basin to the Merced, and are also found in northern California streams such as the Trinity and Salmon. Placer deposits ultimately accounted for 40 percent of the gold extracted in California.

Beginning with the Gold Rush of 1848, rivers in gold country suffered what may have been the greatest devastation ever incurred upon river-scapes anywhere in America. Streambeds and floodplains were literally turned upside down by dredgers, and the high-powered water cannons of hydraulic miners washed whole mountainsides into streams. The resulting silt and debris wreaked havoc on all they touched including salmon spawning beds, downstream wetlands, riparian farmland, and San Francisco Bay. More than a century later the effects can still be seen, especially in rivers where dredging was done at a massive scale such as the Yuba above Marysville, Tuolumne below LaGrange, and Merced near Snelling. In today’s beautiful foothill reaches of the Feather, Yuba, American, Stanislaus, Mokelumne, Tuolumne, and Merced, piles of cobbles still indicate where the floodplain and riverbed were excavated. Sharp rocks, poorly sorted alluvium, absence of mature floodplain forests, and depauperate streamlife all remain the legacy of the often-revered ’49ers as well as the miners that followed. Toxic mercury remains from the processing of the gold. Though thousands of people enjoy rafting, kayaking, and swimming in these rivers today, their fisheries and related life remain compromised by mining abuse. However, tremendous recovery has occurred, and further recovery continues.

On the east side of the Sierra, geologic characteristics of the desert and the rifting of the Great Basin shape the region’s rivers. Historically the Susan, Truckee, Carson, Walker, Mono Lake tributaries, and Owens nourished unique systems of life and eventually evaporated in vast landlocked lakes. Today the rivers are heavily tapped and depleted before they reach their ends, and their historic lakebeds are exposed by receding water levels. With no outlets, the remaining runoff becomes more and more saline as water evaporates. Mono and Pyramid lakes are two lakes now being re-nourished with the flows they need, at least to a partial extent.

Beyond the reach of Sierra streams, the extremely arid valleys of California’s southeastern desert are landlocked; most runoff sinks into alluvial fans, though flash floods surge briefly in rivers that empty onto alkali playas. During the ice ages the desert rivers carried vast surging glacial runoff from the Sierra Nevada, most of which drained into Death Valley through a vast network of lakes and linked rivers.

In geologic time, the western base of the Sierra Nevada was once the Pacific Coast, and lowlands there subsided and filled with silt washed down from the mountains, eventually accumulating to a depth of 20,000 feet, and the inland sea became today’s Central Valley. Later, the Coast Range uplift blocked the Sierra rivers from former exits to the Pacific, leaving them to accumulate in the north–south alignment of today’s Sacramento and San Joaquin rivers. With the flush of rain and snowmelt, these arteries historically ponded into extensive wetlands, riparian forests, and plains of native grasses so fruitful they were called an American Serengeti. The Sacramento and San Joaquin now join and advance out to sea through the Golden Gate—the only remaining gap that completely penetrates the Coast Range south of the Klamath.

California’s southern mountains, including the Coast, Transverse, and Peninsular ranges, arose as a result of tectonic activity along the San Andreas and related faults. These precipitous, fractured, youthful ranges are cut by steep-gradient streams that show some of the most vivid examples of raw erosion in America. Loosened by earthquakes and exposed to periodic fire, enormous volumes of soil, rock, and mud are flushed down southern California’s steep watercourses when the mountains are pounded with rain.

Climate

The climate that delivers the water determines the flow of streams everywhere.

This all starts with the Pacific High—a vast weather system that governs California’s weather. It originates with the constant warming of air in the tropics. That hot air rises in elevation over the equator until it cools and spreads north and south. Then, being colder and heavier, it descends once again as a high-pressure block at mid-latitudes. All summer—and in much of spring and autumn—the Pacific High pushes cloudy low-pressure systems away from California and keeps the skies clear. With autumn’s changing orientation of the earth relative to the sun, and subsequent winter cooling at California’s latitudes, the high-pressure belt migrates south, allowing wet low-pressure fronts to approach from the Pacific. This annual cycle gives the state a Mediterranean climate—dry hot summers and cool wet winters.

Following temperature gradients, the Pacific High is strongest to the south and grows weaker to the north, and so the climate is progressively wetter to the north. All the North Coast rivers benefit from heavy winter rains and a long wet season. The rivers rise and fall in cadence with the storms, and on queue, the salmon and Steelhead enter from the ocean and swim upstream to spawn. Even in the north, summers are dry, and so rivers drop to a fraction of their winter levels. Plants that require year-round moisture, such as redwoods, are forced to depend on fog or on groundwater present on floodplains. The Sierra Nevada rivers are also affected by the northern storm phenomenon: 80 inches of precipitation is common in the northern Sierra while southern peaks typically get 40 inches, and the rivers respond accordingly.

With a shorter rainy season and smaller storms, coastal southern California receives about 15 inches of rain a year; rivers there are small and easily depleted during the dry season. Throughout California, extended droughts can last for several years. Likewise, some years have torrential amounts of rain and flooding, peaking from November through February.

The hemispheric influence of the Pacific High and its adjacent low-pressure systems is the master governor of precipitation, but mountain ranges also acutely influence rain and snow. The western, windward sides of California’s north–south ranges force saturated clouds blowing in from the Pacific to rise and cool, and the atmospheric vapor condenses into rain. In some areas annual precipitation increases a full inch for every 50 feet of rise in the Coast Range. Mountainous areas thus generate far greater runoff than lowlands. The leeward (downwind) sides of the ranges experience a rain-shadow effect; because the air warms when descending, it retains its water vapor more effectively and yields less precipitation. This results in low stream volumes on the east side of the Coast, Peninsular, Cascade, and Sierra Nevada ranges.

The higher the mountains, the greater the elevation (orographic) effect on precipitation (up to the point where the moisture within the air mass is largely spent). Rising to 7,000–14,000 feet, the Sierra Nevada is one of the continent’s premier moisture-raking topographies with snowfields reaching consolidated depths of 12 feet by spring. Even once the weather warms, this snow takes a long time to melt, and so the snow feeds the rivers through midsummer or longer. Likewise, high summits of the Klamath Mountains, Mount Shasta, and Lassen Peak accumulate tremendous depths of snow that melt slowly and nourish local rivers directly or through groundwater. Mountains of the Central and South coasts get deluged with occasional winter storms, but most of this is rain, so the water runs off the steep, rocky, sparsely forested slopes quickly, leaving the streams dry for most of the year. The tallest mountains of the Central and South coasts are high enough to accumulate at least some snow, and these peaks are the sources of most of the region’s perennial streams.

Light precipitation in the south due to the dominance of the Pacific High combined with rain shadows results in extremely arid conditions inland. The Mojave Desert receives about 5 inches of rain a year and has no perennial rivers except for one short section of the Amargosa and a few palm oasis streams east of the San Jacinto Mountains. Penetrating this arid landscape, the Colorado River brings its voluminous flow the entire distance from the Rocky Mountains, 1,400 miles away. North of the Mojave, the Great Basin Desert likewise gets scant water other than the runout of the eastern Sierra streams.

Snow accounts for most of the river runoff in the Sierra Nevada. A late-season storm covers the shores of the North Fork Stanislaus above Spicer bridge.

The massive snow accumulations and colder temperatures of the ice ages created glaciers across much of the upper Sierra Nevada and in the highest mountains of the north. Inching their way downward in slow but plastic movement, the glaciers scraped and gouged their way to mid-elevation valleys such as Yosemite and Cedar Grove, creating the spectacular scenery and river courses we know today. U-shaped canyons signify former glaciation, while V-shaped canyons indicate erosion entirely by water. The glaciers carried rock that had fallen onto the ice from mountainsides above and ridden the slow-moving ice, as if on a conveyor belt, to the terminus where the ice stopped advancing and the rock settled as terminal moraines. These barricaded the streams behind them, and remnants of moraines stand today as significant features that dam high mountain lakes, create rapids with boulders they’ve pushed into place, and contribute gravel that blankets floodplains far below.

During the ice ages, glacial melt in landlocked basins east of the Sierra formed the ancient Lake Lahontan, larger than New Jersey and covering much of northeastern Nevada. The terminal lakes of Pyramid and Walker are now supplied by the Truckee and Walker rivers, respectively, but represent just the lowest lying remnants of that vastly larger ice age lake. Lahontan Cutthroat Trout evolved as a distinct species found only in the rivers of the giant glacial meltwater basin; vestigial populations of these endangered fish survive in a few streams even though the lake is long gone.

With runoff that persists all summer, shrinking glaciers still contribute flows to the Walker, Owens, Sacramento, Shasta, and Trinity basins, often by way of groundwater discharges. Because of global warming, the days of California’s glaciers are numbered, except perhaps on Mount Shasta, which for the time being is high enough and sufficiently far north to receive large amounts of snow instead of rain.

Dramatic year-to-year variations in the weather have been a fact of life throughout California’s history, but the climate is now changing with warmer temperatures worldwide, and the ramifications for rivers are extreme. Climatologists predict that more precipitation will come as rain and less as snow, which will result in more winter floods, lower summer runoff, and a sharp reduction of the snowpack that now stores water so effectively. The Sierra snowpack may exceed the content of the region’s reservoirs, but the total snowpack will probably be reduced by 50 to 80 percent within this century. Because of climate change, water supplies for cities and farms will be reduced and natural ecosystems will be heavily stressed (see The Problems, Protection, and Restoration of California Rivers).

Hydrology

The timing, volume, and other qualities of rivers’ flows define the character of waterways and are critical to all the life within them.

In northern California, rivers are principally fed by rainfall in winter, yielding big flows that can occur October through April; storms cause floods especially from November to February. Salmon and Steelhead wait at the mouths of streams, and when the first high water comes, they enter the rivers on their spawning runs. This is when they can be seen or caught while finning their way up riffles or waiting in clear pools for spawning opportunities. The high water likewise creates superb whitewater for kayakers, rafters, and canoeists. The smaller streams of the Central and South coasts likewise swell when nourished by winter rains, and some channels open up to remnant runs of Steelhead.

While enormous flows flush off the northern mountains during winter rains, the extended river flows in California come from snowpack. This takes time to melt, and so the snow-fed rivers generally rise more slowly than the rain-fed rivers and subside more slowly. They peak from April to June, months after the precipitation occurs. Seeing the high waters of the north usually requires going out in the rain, but experiencing the flush of runoff in the Sierra is done in summer sunshine.

The higher elevations of the northern ranges, the tallest peaks in the south, and the Sierra Nevada in its entire length accumulate vast amounts of snow. Fed principally by snowmelt, Sierra streams from both the east and west sides of the range contribute 52 percent of all the runoff in California, together topping even the volume of the rainy North Coast. In the northern Sierra, the Feather carries twice the volume of the next-largest Sierra river, the American, which is the next basin southward. In the southern Sierra, the storms drop less snow but the elevation is higher, so the snow lasts a long time and continues to deliver water even in late summer.

Sierra meltwater typically peaks in late May or early June, though it’s coming progressively earlier. Extended summer flows are found in rivers with the highest basins and in streams draining north faces of mountains. They provide critical cold runoff to rivers that would otherwise warm up in summer. These waters will be increasingly important to freshwater ecosystems in the warming climate of the future.

Saturated soils continue to discharge water and maintain sizable flows for weeks or months after the rains and snowmelt stop, though this spongelike effect has been reduced by watershed disturbance. Logging, farming, grazing, and land development all eliminate or alter the porous and organic surface layers of soil. Even worse, roadcuts intercept subsurface water moving through the soil and sluice it away in concentrated form and in ditches that cause erosion.

Some water sinks deeper beneath the soil to become groundwater, which slowly seeps through underground cavities, gravel deposits, and water-permeable formations such as sandstone, limestone, and basalt. Except for deep groundwater and low-elevation aquifers, these flows reemerge as springs, which feed streams longer than does surface runoff. In the Central Valley, deep deposits of gravel and soil hold enormous quantities of groundwater that accumulate when the rivers spill over their banks. However, flood-control dams, levees, channels, and diversions have reduced groundwater recharge, and with excessive pumping for irrigation, underground reserves stored up over the millennia are rapidly receding.

The seasonal variation of flows is a crucial part of a river’s natural cycle. High water dislodges silt, sand, and gravel from channels and banks and washes it downstream. The river then deposits this bed load on the insides of bends where the current is slower, and it erodes yet more material on the outsides of bends, constantly renewing the shape and condition of the shores. The biological health of floodplains and riparian forests depends on this cyclical