Channel geomorphology and riparian vegetation development following dam construction on the Bill Williams River, AZ, USA

Background/Question/Methods

In dynamic river systems, riparian vegetation colonization and community development are profoundly affected by physical forces, including flood hydraulics and sediment transport. Vegetation communities, in turn, may alter local erosion and deposition patterns by increasing drag forces and soil cohesiveness, and ultimately affect floodplain and river morphodynamics. These interactions can be particularly strong in semi-arid ecosystems, where physical forces dominate, vegetation influence is highly variable, and large dams can alter the balance of these processes. Our research quantified the effects of riparian vegetation type and density on channel geomorphic change along the Bill Williams River, AZ (Southwestern U.S), a sand-bed river that was dammed in 1968.  We digitized changes in channel course and vegetation community structure from 1953 to 2009, at 3-12 year intervals along a 65-km reach. The vegetation community structure consisted of cover type (cottonwood/willow, tamarisk, and others) and density (dense and sparse patches). Geomorphic indicators include channel width, sinuosity, braiding index, and lateral migration rate.

Results/Conclusions

Following dam construction, the annual peak flood magnitude on the Bill Williams River declined by 91% to 36 m³s^-1 on average. Concurrently, channel width decreased by 70% and the channel’s lateral migration rate by 68%. The area dominated by tamarisk increased from 21% to 33% and native cottonwood and willow from 14% to 19%, indicating vegetation encroachment along river corridor, especially tamarisk. In contrast to other studies that found river channel simplification after dam construction, the Bill Williams River sinuosity and braiding index increased by 3.6% and 19%, respectively. These trends may be caused by vegetation effects on diverging flow and increasing river branching. At a subreach scale (500 m segments), the channel width and channel lateral migration were negatively correlated with the surrounding vegetation density. In particular, the effects of tamarisk patch density on channel width were 17% greater compared to the overall vegetation density, suggesting some riparian species can accelerate channel modifications. Although river geomorphology responded strongly to changes in flood energy following dam construction, vegetation also played an important role in altering river geomorphology at different temporal and spatial scales.  Therefore, the predictions of channel evolution should take into account the riparian vegetation structure, including both species and density or abundance.