California's Coastal Streams and Rivers

The following materials are exerpts from the California Coastal Commission's California Coastal Resource Guide, which can be ordered from University of California Press by calling 1-800-822-6657.

One their way to the ocean, California's coastal streams and rivers flow through the canyons and valleys of coastal mountains, linking forest, chaparral, scrubland, grassland, and marsh. Riparian woodlands develop along stream banks and floodplains, and coastal wetlands and estuaries form where the rivers enter the sea. Rivers transport nutrients, sediments, and oxygen through the watershed, and life flourishes in their path.

Streams and the surrounding riparian woodlands support numerous animal species, including, frogs, salamanders, snakes, muskrats, beavers, and river otters. Spruces, maples, cottonwoods, alders, and willows grow along the stream banks and attract large numbers of resident and migratory birds. An entangling understory of shrubs, flowering plants, and vines provides sites for nesting, shelter and shade for many animals. Algae and mosses proliferate in the water and on rocks. Leaves swept into the current decompose, adding nutrients and organic matter. Insect thrice here and in turn provide an abundant food source for invertebrates, fish and birds. Anadromous fish such as salmon and steelhead migrate from the sea to fresh water to spawn, and depend on well-oxygenated streams and gravelly streambeds and spawning sites.

The largest coastal rivers are found in Northern California, where sixty percent of California's annual rainfall occurs. The Klamath River whose watershed drains 12,000 miles, is the second largest river in the state, after the Sacramento River. Flowing through the northern redwood and fir-forested mountains to the coast, the Klamath, Trinity, Salmon, Smith, Eel, and Van Duzen rivers are protected by the Wild and Scenic Rivers Act of 1972. The Act preserves the free-flowing natural state of rivers by prohibiting dam construction and diversion structures.

River runoff, the amount of water discharged through surface streams, is determined by a combination of factors, including local geology, topography, drainage area, and rainfall patterns. As rainfall and moisture diminish southward along the California coast, runoff decreases, and rivers are accordingly smaller in size. In Southern California, rivers and streams only maintain year-round flows near their headwaters. At river mouths, groundwater and agricultural runoff may provide the only source of water flow in the summer.

As they flow down from their headwaters toward the coast, rivers carve steep, narrow canyons through the mountains. As they approach the coast they lose speed, depositing sediment to build broad floodplains with rich, deep soils. Large river floodplains, such as the Eel River Delta, the Salinas River floodplain, and the Oxnard Plain, tend to be productive agricultural regions, and are often centers of urban and industrial development. The Los Angeles Basin, formed by the Los Angeles, San Gabriel, and Santa Ana rivers, contains California's largest urban population. Because river lowlands are subject to periodic flooding, streams in populous regions are often contained within levees or concrete channels to limit their potential destructive force.

Coastal rivers play a crucial role in replenishing sand lost from beaches. In Northern California, the larger, perennial rivers carry sediments eroded from their upper watersheds to coastal beaches throughout the year. But in Southern California, where rivers run intermittently, stream sediments reach the beaches only during large storms and floods. Dam construction and urban development along these rivers have reduced their natural sediment loads, resulting in serious sand supply and erosion problems on south coast beaches.

Dam construction, channelization, water diversion projects for agricultural irrigation, and the increased water demands of growing urban areas have dramatically diminished the size of many California rivers and reduced the diversity of species associated with riparian habitat. By decreasing water flows, a dam can transform a perennial stream into an intermittent one, block the path of migrating fish, damage fish spawning gravels, deprive estuaries of needed fresh water, and reduce sediment nourishment of beaches.

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