Late Holocene sediment storage in canyons of the Colorado Plateau; with Suppl. Data 87-27

Field surveys of ten drainage basins 10 to 1,790 km2 in area in the Colorado Plateau (Utah and Arizona) and repeat photography of 156 fluvial environments providing comparative evidence spanning up to 114 yr reveal that two stratigraphic units characterize the alluvial materials on valley floors. An older unit deposited generally between 1250 A.D. (+ or -200 yr) and 1880 A.D. (+ or -20 yr) was dissected by arroyo development by the early twentieth century. A younger unit was deposited generally during the period after approximately 1943 (+ or -5 yr). Rates of sedimentation in the 1943-1980 period were almost twice the rates in the 1250-1880 period. Channel-storage rates during the two periods of sedimentation are related to drainage-basin area by a power function similar to functions relating slope-sediment production and sediment yield to drainage basin area. Storage rates in the 1943-1980 period were about 50% of the rates of slope-sediment production. The amount of sediment stored in the two units exhibits a definable spatial pattern. Source streams (with basin areas upstream < 1 km2) consistently passed sediments eroded from slopes through the channel system with little storage. Local streams (with basin areas upstream of 1-1,000 km2) alternatively stored and then evacuated large amounts of sediment in apparent response to climatic and land-use changes. Regional streams (with basin areas upstream of 1,000-10,000 km2) stored or evacuated moderate amounts of sediment in response to events upstream. Inter-regional streams (with basin areas upstream > 10,000 km2) alternatively stored or evacuated moderate amounts of sediment in response to upstream evacuations of sediment from the Colorado Plateau, riparian vegetation change, and hydro-climatic changes in mountain watersheds outside the plateau region.