THE EFFECTS OF SUBSTRATE VARIABILITY AND INCISION ON THE DOWNSTREAM-FINING PATTERN IN THE COSUMNES RIVER, CENTRAL VALLEY, CALIFORNIA BY CANDICE RAY CONSTANTINE B.S. (Tufts University) 1999 THESIS Submitted in partial satisfaction of the requirements for the degree of MASTER OF SCIENCE in GEOLOGY in the OFFICE OF GRADUATE STUDIES of the UNIVERSITY OF CALIFORNIA Davis Approved: ________________________________________ ________________________________________ ________________________________________ Committee in Charge 2001 1 Abstract Downstream fining of gravel in alluvial rivers is attributed to the processes of selective sorting and abrasion and is typically modeled as an exponential decrease in median grain size over the length of the profile. Exponential downstream fining produced through selective sorting has been linked to profile concavity and declining bed shear stress. Downstream-fining patterns are sensitive to changes in sediment supply and flow regime and can be expected to change in response to human activities that alter these variables. Over the last 70 years, flood-management practices in the Cosumnes River, California have influenced flow regime and induced incision in low-gradient reaches downstream of Highway 16. It is probable that changes in flow and the thalweg profile have been accompanied by adjustments in the downstream-fining trend, and ongoing geomorphic evolution will continue to impact the size distribution of bed material. A geomorphic survey was conducted in order to define the existing downstream-fining pattern in the study area and document the current controls on geomorphic evolution. The survey consisted of 39 bed material samples, a 42.8 km thalweg profile, and 30 cross-sections located between Highway 16 and Twin Cities Road. Data was also used to determine the processes responsible for formation of the downstream-fining trend. Although reach-scale variations in slope exist, the overall slope of the upstream 30 km of profile is nearly constant, and bed shear stress does not decline exponentially with distance downstream. Despite the lack of profile concavity, the downstream-fining pattern in the study area is best described by an exponential function. Results demonstrate that downstream fining is the aggregate effect of selective sorting that operates discontinuously over the length of the profile rather than the product of abrasion. Sorting and fining occur in reaches where bed shear 2 stress is low relative to the critical shear stress required for entrainment of the surface median. Where bed shear stress is high, the channel bed experiences frequent scouring and all grain sizes are transported downstream. Median grain size does not decline exponentially with distance through these reaches as it does elsewhere. Bed shear stress and the potential for sorting are determined by local slope and depth which are largely controlled by geology in many reaches. Local-scale sorting processes may contribute to downstream fining in the study area and in other systems where slope is longitundinally constant. The overall slope of the profile declines significantly after about 30 km and generally remains low for the final 10 km of the profile. Decreased slope and lowered maximum channel depth are achieved where the river exits a long segment bounded by levees and enters one where levees are set back from the channel and overbank flooding occurs annually. Diminished bed shear stress beginning at this transition accounts for the change in bed material from gravel to sand which requires approximately 8 km for completion. With the existence of the baseline information provided by this study, future surveys can be used to assess the effects of contemporary geomorphic response and future restoration efforts on downstream fining. 3 Acknowledgements I would like to first thank my husband, José, for his enthusiastic support and encouragement. His love and patience provided me with confidence, and his questions and thoughtfulness with interesting conversation and new ideas. I would also like to thank all of my friends and family members who cheered me on. A huge thanks is due to the members of my committee: Dr. Jeffrey Mount, Dr. Joan Florsheim, and Dr. Gregory Pasternack for their constructive criticism and advice. In particular, I thank Jeff for his ideas and challenges and extend a special thanks to Joan for her patience and willingness to share her knowledge of the Cosumnes. Their interest in the project and desire to see me become a better scientist never went unnoticed. I would like to send a giant thanks to my field assistants who, usually ungrudgingly, endured long, hot days in the Cosumnes River. Above all, Andrew Orr, Lily Wong, and Elizabeth Stevick deserve my gratitude. Without their readiness to tread water while holding the stadia rod, I would never have completed 42 km of profile. I sincerely thank José Constantine, Devin Taylor, Marie Reil, Matt Marlatt, Sharon Ruth, Lee Higgins, Sarah Yarnell, Kathryn Thomas, and everyone else who helped out for the invaluable field and lab assistance. I extend my thanks to everyone at the Cosumes River Preserve and to Cosumnes River landowners who graciously allowed me to access the river through their properties. This research was made possible by support from the UC Davis Department of Geology, CALFED Bay-Delta Ecosystem Restoration Program (99N06), and David and Lucille Packard Foundation Conservation Program (1988-3584). 4 Introduction Reduction in the size of bed material with distance downstream is commonly observed in gravel-bed rivers. Termed downstream fining, researchers have attributed this feature to the processes of selective sorting (Ashworth and Ferguson, 1989; Ferguson and Ashworth, 1991; Paola et al., 1992; Ferguson et al., 1996; Seal et al., 1997) and abrasion (Bradley, 1970; Schumm and Stevens, 1973; Werrity, 1992; Kodama 1994a, 1994b). Studies in rivers with concave profiles have emphasized the role of declining slope in producing downstream fining, especially where lithologies are relatively resistant to abrasion (Parker, 1991; Paola et al., 1992; Hoey and Ferguson, 1994; Ferguson et al., 1996; Hoey and Ferguson, 1997; Seal et al., 1997). Decreasing slope implies diminishing flow competence, or ability to transport a given particle size, which leads to selective deposition of coarse grains upstream and preferential transport of fine grains downstream. Downstream-fining patterns which reflect sediment-transport conditions are necessarily sensitive to spatial and temporal changes in flow regime and sediment supply (Wolcott, 1988; Hoey and Ferguson, 1994; Pizzuto, 1995; Hoey and Ferguson, 1997; Hoey and Bluck, 1999). Therefore, changes in flow and sediment supply induced by anthropogenic activities such as flow regulation, levee construction, channelization, and land-use practices can be accommodated through adjustments in fining patterns. These adjustments or modifications in the nature of bed material have important consequences for aquatic life. Salmonid species, for example, require specific substrate conditions for spawning (Bjornn and Reiser, 1991). Native salmonids return annually to spawn in the Cosumnes River, California; however, populations have declined over recent years. Knowledge of the processes that alter grain-size patterns and thereby affect the quality and distribution of riparian habitat is necessary for 5 successful restoration of native fish species in the Cosumnes and other California rivers. Unlike most western Sierran rivers, the Cosumnes has no major dam on it to regulate flow; thus, it presents a unique opportunity to examine the effects of anthropogenic change on fluvial geomorphology and related habitat suitability in the region. This field study focuses on the influences of substrate variability and profile shape, products of recent incision, on the downstream-fining pattern exhibited by the Cosumnes River. The study area is a low-gradient segment of the Cosumnes River from approximately Highway 16 downstream to Twin Cities Road. Where downstream fining is dominated by selective sorting, the degree of fining should correlate with the degree of profile concavity (Parker, 1991). In other words, a sharp decline in median grain size over a short distance should be accompanied by significant decreases in slope and bed shear stress. In contrast, median grain size should not show substantial downstream reduction where the channel profile is linear. Bed material in the Cosumnes is relatively resistant; therefore, it is expected that selective sorting caused by declining bed shear stress dominates the production of downstream fining in the study area. In order to document the downstream-fining pattern in the lower reaches of the Cosumnes River, 39 surface and subsurface bed-material samples were collected over 42.8 km. A thalweg profile was surveyed over the same distance to record downstream changes in slope which influence selective sorting. Despite a lack of strong profile concavity in the study area, bed material exhibits an exponential downstream-fining trend and undergoes a transition from gravel to sand. Thirty cross-sections completed in the upstream 32 km of the study area provide information about bankfull depth and channel form, characteristics which also affect sorting processes. Results suggest that exponential downstream-fining patterns can develop through 6 selective sorting in river segments where longitudinal concavity is weak and sorting is not continuous over the length of the profile. Background Downstream