Surface erosion also adds to sediment production and is influenced by parent material, slope, and vegetative cover. Excluding the coastal plain, 90% of the basin has high or extreme erosion potential (California Department of Fish and Game 1980). Steep slopes generally erode faster than gentle slopes for several reasons. Steep areas are less likely to build up soil and organic debris and therefore have less absorptive capacity. Because less water is absorbed, more water flows overland and erodes more material.

Dense vegetation helps prevent erosion by providing protection from raindrop impact and increasing storage and infiltration of precipitation. Conversely, removal of vegetation usually increases overland flow and the effect of raindrop impact. Loss of vegetative cover increases surface erosion and sediment yields.

Erosion on many abandoned or unmaintained roads is causing chronic sediment input to streams. In particular, erosion is occurring on mining roads such as those associated with the Cal-Nickel explorations on Gasquet Mountain. Off-highway vehicle use of these areas further accelerates erosion.

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