An evaluation of land use, hydrology, and sediment yield in the Mill Creek watershed, northern California

Madej, Mary; O'Sullivan, Colleen; Varnum, Nick

Conditions in the Mill Creek watershed lead to a lower rate of erosion and fewer landslides than in other basins of Redwood National Park. Although some aggradation has occurred in recent years, old-growth redwoods in the one-third of the basin that is within state/federal parks are situated high enough not to be threatened by present amounts of sediment. The report describes the climate, vegetation, geology, soils, and physiography of the basin, along with land use history, erosional features, cross section surveys, hydrology, and sediment yield. The Mill Creek watershed (100 km 2 ) in northwestern California has undergone timber harvest since the 1850's. Two-thirds of the basin is privately owned and, as of 1984, 60% of that portion has been logged. The remaining third of the basin is under state and federal park management. Most of the Mill Creek basin is underlain by competent sandstone of the Franciscan Assemblage. Sediment yields from the basin are lower than from nearby basins that are composed of more erodible bedrock. Total load in Mill Creek is about 140 t/km 2 , of which 60% is suspended sediment, 10% bedload, and 30% dissolved load. In Mill Creek, a tributary of the Smith River, mean annual runoff is 180 cm and mean annual precipitation is 266 cm. The 1.5, 10, 50 and 100-year recurrence interval floods at the Mill Creek gaging station (A=74.1 km 2 ) are 70, 115, 140 and 150 m 3 /s respectively. Mill Creek runoff closely mimics that of the Smith River, and the latter can be used as a surrogate for Mill Creek when the Mill Creek gage is not in use. Cross section surveys from 1974-1985 show recent scour following deposition in the 1970's. Over a meter of aggradation occurred at the mouth of Mill Creek since 1930. The channel changed most dramatically during high flow years. Bank erosion was common, but not severe, along the upper reaches of Mill Creek. Small slope failures are also common, but large-scale earthflows and debris slides typical of nearby basins are rare in Mill Creek. The total volume of sediment (active channel sediment plus stable sediment) stored in Mill Creek on a per unit basis is 74,000 m 3 /km 2 . This is over three times the amount of sediment stored in Redwood Creek. However, active channel sediment represents only one percent of the total stored sediment in Mill Creek. Abundant low-lying terraces along Mill Creek help protect the base of hill slopes from erosion, resulting in fewer streamside landslides than in adjacent basins. Old growth redwood groves located in park lands are situated on terraces well above the area affected by recent aggradation and are not currently threatened by channel changes in Mill Creek.

An evaluation of land use, hydrology, and sediment yield in the Mill Creek watershed, northern California

Series

Collections