DIRECT AND INDIRECT BIOLOGICAL IMPACTS OF SEVERE SEDIMENT LOADING IN A GRAVEL-BED RIVER

Widespread landsliding, gullying, and road failures initiated during large storms in the 1960s and 1970s resulted in many meters of channel bed aggradation in Redwood Creek, northwestern California. Thirty years of channel monitoring has tracked the fate of this excess sediment and has documented direct and possible indirect impacts on aquatic and riparian habitat. Concurrent monitoring by California Department of Fish and Game has documented a decrease in salmonid populations in the watershed during this period. Streamside old-growth redwood trees were damaged or killed by gravel deposition, leading to reduced shade canopy over the streambed and a shift to a hardwood-dominated riparian zone. The channel became wider and shallower, leading to increased summer stream temperatures, and possibly increased algal production. Aggradation filled pools, important rearing habitat for salmonids, reducing the volume of this habitat type. As pools became shallower, the thermal stratification once observed in deep pools, which provide cool water refuge to fish, became less common. Hyporheic flow patterns may have changed as well as bed relief became less pronounced. In addition, more flow goes subsurface into the gravel deposits during summer base flows, leading to lower surface water availability and even dry reaches. The dominant particle size of the channel bed decreased and sand facies became more frequent, possibly leading to a shift in benthic macroinvertebrate communities. Aggradation in lower Redwood Creek has persisted for more than 30 years, affecting multiple life cycles of salmon. Recently, however, in upstream reaches where the river has removed most of the flood deposits, freshwater mussels have been observed for the first time in decades.