The Napa River is a river approximately long in the U.S. state of California. It drains a famous wine-growing region called the Napa Valley, in the mountains north of the San Francisco Bay. Milliken Creek and Mt. Veeder watersheds are a few of its many tributaries. The river mouth is at Vallejo, where the intertidal zone of fresh and salt waters flow into the Carquinez Strait and the San Pablo Bay. Course The Napa River rises in northwestern Napa County just south of the summit of Mt. St. Helena in the Mayacamas Mountains of the California Coast Ranges. The source begins as seasonal Kimball Canyon Creek in Robert Louis Stevenson State Park at an elevation of which descends the southern slope of Mt. St. Helena to Kimball Canyon Dam. It flows south for 4 miles (6 km), entering the head of the slender Napa Valley north of Calistoga. In the valley, it flows southeast past Calistoga, St Helena, Rutherford, Oakville and through Napa, its head of navigation. Downstream from Napa, it forms a tidal estuary, entering Mare Island Strait, a narrow channel on the north end of San Pablo Bay. It discharges into San Pablo Bay through the Napa Sonoma Marsh. Watershed The Napa River watershed encompasses approximately 426 square miles (1,103 km²). Larger tributaries, such as Dry, Conn, and Soda creeks, show signs of recent incision and have graded to the incised current level of the mainstem Napa River. In some cases, smaller tributaries cutting across the valley floor have not fully adjusted to the lowered level of the mainstem and are elevated at their confluence with the mainstem, forming potential barriers to upstream fish migration. Several large dams were built between 1924 and 1959 on major eastside tributaries (Conn, Rector, Milliken, and Bell dams) and the northern headwaters of the Napa River (Kimball Dam). In addition, many smaller dams can be found throughout the watershed. These numerous dams are impassable barriers to salmon and steelhead seeking their historic spawning grounds. Ecology The river also supports a remarkable diversity of fishes and recovering salmonid populations, especially chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss). In 2003 the Napa County Resource Conservation District began an ongoing salmon monitoring program, and have recorded a run of approximately 400 - 1000 fall-run Chinook salmon the past several years. The Chinook run begins in late October through January. Conclusive evidence of historical