DYNAMICS OF SEDIMENT TRANSPORT AND STORAGE DURING CYCLES OF AGGRADATION AND DEGRADATION IN PROXIMAL AND DISTAL REACHES OF SEDIMENTARY SYSTEMS: TWO FIELD AND FLUME EXAMPLES

Sediment passes through fluvial systems under channel conditions that respond to sediment load and affect transport capacity in ways that are poorly understood. Here we report results from two paired field-flume studies of the effect of sediment supply on transport and storage during cycles of aggradation and degradation in gravel-bed channels. Field measurements provide a partial record of changes in bed elevation, channel morphology, and armoring. Flume experiments modeling the field prototypes reveal processes that could not be observed closely in the field. After a well-armored channel was formed in equilibrium with a low feed rate, cycles of aggradation and degradation were forced by varying the sediment feed rate while holding discharge constant. Cuneo Creek in northern California represents proximal reaches near large sediment sources that have pronounced variations in sediment supply, steep slopes, and poorly sorted sediment. Redwood Creek in northern California represents distal reaches downstream of large sediment sources, low slopes, and superior sorting. Results show that relative rates of transport and storage inducing bed elevation change are influenced by variations in channel morphology and armoring. In the proximal case, widening and braiding during advanced aggradation suppressed increases in transport rate and enhanced deposition. During degradation, a single-thread channel formed and rapidly incised, transport rates increased, and the bed later armored, producing negative hysteresis in transport-storage relations. In the distal case, morphologic changes during aggradation were moderate, and variations in surface texture were primarily responsible for changes in transport and storage. Results indicate that enhanced aggradation in proximal reaches could create high flood terraces during sediment pulses and temporarily store large sediment inputs. Distal reaches receive more dispersed pulses and undergo smaller changes in bed elevation and channel morphology. The concept of dynamic transport capacity may open new approaches to studying fluvial landforms, sediment routing, and drainage network evolution.