Limiting Riparian Hardwood Encroachment Along the Trinity River, California

Regulated rivers, as the Trinity River, often lose flow variation and are encroached by riparian vegetation. Riparian hardwood germination and establishment patterns on channel rehabilitation sites and the effects of maintenance flow releases from Lewiston Dam were quantified. On the Trinity River, riparian hardwoods germinate within 0.5 meters of summer baseflow water surfaces on substrates of coarse sand or finer. The upper germination limit corresponds to the edge of soil surface moisture 0.14 meters above the June 6, 1995 water surface. Channelbed scour within the active channel is attained by floods of 170m³/sec or greater. Riparian hardwood establishment is restricted to those parts of the channel not experiencing channelbed surface layer scour. Bank rehabilitation site re-encroachment has been prevented by annual channelbed scour, which has eliminated 90–100% of establishing seedlings. Within three years of a riparian hardwood's germination, maintenance flow releases must be of at least 170 m³/sec to scour the channelbed surface and restrict encroachment.