John J. Berger

The Mattole River (Figure A.8) rises south of the town of Whitehorn in Northern California's Humboldt County and flows 62 miles northwest to the Pacific Ocean, which it meets 8 miles south of Cape Mendocino (Mattole Restoration Council [MRC], 1989). About 2,000 people inhabit the 306-square-mile watershed, which has a generally mild, Mediterranean climate due to its proximity to the ocean (MRC, 1989; House, 1990).

Mountain ridges and peaks on the western side of the Mattole watershed uplift winter storm clouds from the ocean and produce frequent heavy rains. Precipitation usually occurs in the winter months and ranges from an average of 50 inches (1,270 mm) of rain in the lower watershed to between 80 and 90 inches (2,032-2, 286 mm) in the upper watershed (MRC, 1989).

Measurements of stream flow made since 1950 at Petrolia in the Lower Mattole watershed indicate an average annual flow rate of 1,340 ft ³/s and an average monthly winter flow of 1,710 to 4,170 ft³/s (MRC, 1989). Summer and fall average flows are less than 60 ft³/s (MRC, 1989). The minimum recorded flow was 20 ft³/s, and the peak flood was 90,400 ft³/s (MRC, 1989).

The watershed is in a seismically active area subject to rapid tectonic uplift and high rates of natural erosion and sedimentation (MRC, 1989).

The river bed drops an average of 22 ft per mile in elevation. The Upper Mattole has a relatively steep gradient and stable bedrock in much of the channel. The Middle Mattole is less steep, but the river and streams cut through highly erodible fractured sandstones and decomposing shales and clay (MRC, 1989). The Lower Mattole has a gentle gradient (less than 11 ft per mile) and meanders through a broad alluvial valley to end in a short estuary that becomes a lagoon when reduced summer river flows allow the river mouth to become plugged with sand (MRC, 1989). The gradual river slope results in deposition of gravel bars, islands, and sediment terraces (Focus, 1990).

The Mattole River was once much cooler, deeper, and narrower than it is today. In the 1940s, the river was still shaded by dense forest of Douglas fir, redwood, and native hardwoods (MRC, 1989). The watershed was rich in riverine fur-bearing wildlife, including fisher, mink, otter, and weasel, which were trapped commercially.