Quantifying the connections between flow, bar deposition, and meander migration in large gravel-bed rivers

Predicting channel migration is a key component of designing plans for river management and restoration. Fundamentally, the lateral migration rate depends on the rate of excavation of the outer bank toe. This rate is a function of bank resistance to erosion, flow velocity, bend geometry, and sediment transport and storage. Although it is certain that all these variables play some role in determining migration rates in gravel-bed rivers, the importance of sediment storage in particular has not been thoroughly investigated. Currently employed meander migration models are driven by curvature effects on near-bank flow and account for differences in bank erodibility through use of a calibrated bank erosion coefficient. Despite increasing application of such models to sensitive management problems, it remains unclear how the coefficient varies with properties of the channel and to what extent it incorporates the effects of changes in bed-material storage on migration rate.; In this thesis, I examine meander migration rates and the link with sediment transport and storage in the Sacramento River using a combination of GIS, field data, and numerical modeling of sediment transport and channel adjustment. The results show that spatial differences in bed-material storage change explain up to 70% of the spatial variability in migration rate. Coupled with the observation that bank erosion frequently precedes bar deposition in the Sacramento River, this results suggests that availability of bed material for deposition presents an important control on migration rates. Given a paucity of bed material, bank erosion would produce a wider channel, causing near-bank velocity and migration rate to decline. Examination of the coefficient of bank erosion used in modeling meander migration shows that for large rivers and over long timescales, the coefficient depends primarily on bank material properties and can be estimated directly from field data on bank resistance to erosion by fluvial shear. This result suggests that vegetation plays a limited role in determining the long-term meander migration rate of large rivers. Although secondary to bank material properties in effect, results indicate that upstream bank-protection structures and the availability of bed material for deposition may also influence the magnitude of the coefficient.