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.

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 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.

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 CentralValley. 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,...