Dam-incuced Changes in Geomorphology and Vegetation Along a Stream in Northern California

Dams are well known for trapping sediment and altering natural flow regimes that affect downstream channel geometry and the distribution of riparian vegetation. While many studies have evaluated pre-dam and post-dam effects, and land-use activity adjacent to the channel on riparian vegetation and channel morphology, few have included GIS mapping and an undammed reference stream to serve as a control for studying responses in an alluvial system. This paper evaluated the effects of Warm Springs Dam (established in 1983) on the variation, magnitude, and directional changes of stream channel geometry and riparian vegetation distribution along Dry Creek and compared the changes to a nearby undammed stream with similar geomorphic and land-use characteristics. Six historical black and white aerial photographs were examined for both streams over a 34 year period prior to the dam's establishment (1942-1976), and a 13 year period after (1987-2000), after being scanned and georeferenced in a GIS. For each year, three stream and riparian features were manually digitized on-screen, including the center of the stream channel, bankfull width, and patches of riparian vegetation, as well as the distance land-use was to the channel. Multi-way statistical analyses evaluated variation in stream length and distance that land-use moved from the channel as well as variation and change in the rate and direction of bankfull area and riparian area. Rating curves and hydraulic geometry exponents used stream gauge measurements that analyzed changes in channel geometry (width, depth and velocity). While mean variation in the reference stream's length and bankfull area remained constant during the 58-year study period, Dry Creek's stream length varied 84% (P=0.02) less (shortened 550 m within the 10.5 km study reach), and bankfull area decreased by 52.5% (P=0.01) after the dam. Riparian vegetation decreased 28.5% (P<0.0001) from 1942-1986 on Dry Creek then increased by 2000 to levels similar to those in 1942. Areas of riparian vegetation in the reference stream consistently increased each year of the study by a total of 38.5% (P<0.0001). Spatial variation in the randomly selected study segments explained most of the variation (40%, P=0.04) and rate and directional changes (53%, P=0.04) in riparian area. Variation in the distance land-use activity moved from the channel significantly explained variation in the rate and directional changes of riparian area and bankfull area on both streams (P=0.006 and 0.004, respectively). Rating curves and hydraulic geometry exponents indicated that Dry Creek's channel bed just below the dam was fairly well armored by 2000; the mid-stream channel bed had lowered a mean 1.02 m (P=0.004) from 1987-2003, and a mean 1.41 m (P<0.0001) near the mouth (22 km downstream from the dam). Dry Creek's sediment starved channel incised and became entrenched allowing vegetation to colonize the less frequently flood prone banks and bars. Even though the dam caused major morphological changes to Dry Creek's channel and distribution of riparian vegetation, land-use activity played an important role in influencing channel characteristics and the presence of vegetation in both stream systems.