ANALYSIS OF SEDIMENT ACCUMULATION RATE AND VOLUME IN THE SAN JUAN RIVER DELTA, LAKE POWELL, UTAH, BASED ON REPEAT AERIAL PHOTOGRAPHY AND GIS

Due to low pool elevations brought about by the ongoing drought, it has been possible to constrain the volume of exposed sediment deposited in the upper reaches of the San Juan River Arm of Lake Powell using aerial photographs and GIS mapping software. Previous work by Condit et al. (1978) and Ferrari (1988) suggests that the total annual sediment accumulation of Lake Powell is between 27,000 and 36,946 ac-ft per year, and that 32-33% of the total sediment volume in Lake Powell occurs in the San Juan River Arm. From this prior work it is estimated that between 8775-12,008 ac-ft/yr of sediment is deposited in the San Juan River Arm. In this study we used repeat aerial photos of the exposed delta complex, combined with pool level data, and pre-lake topography via a surface terrain model in GIS to quantify sediment volume. We estimate that 255,752 ± 5780 ac-ft (315,459,862 m3) of sediment has been deposited in the San Juan River Arm of Lake Powell from Copper Canyon upstream to just below Grand Gulch (River Mile 45-75). This estimate is built upon an assumption of a 1.1-1.325 m/km average slope of the delta complex top surface in 10 of 13 sections analyzed. The remaining 3 sections were assumed to have a slope of 0.0 m/km due to local aggradation. The sediment volume is 76.5% of the pre-lake volume of the study area (334,211 ac-ft or 412,235,900 m3). The study area has been inundated for approximately 29 of the 43 years since Glen Canyon Dam was closed, although inundation periods vary due to pool level fluctuations, causing the delta to oscillate between progradational, aggradational, and retrogradational depositional environments. Based on these observations, it is calculated that the average rate of sediment filling along the entire San Juan Arm is 8,819 ± 199 ac-ft/yr (10,877,926 m3). Based on estimates that the San Juan represents 32.5% of the total sediment volume of Lake Powell (Condit, Et al, 1978; Ferrari, 1988), the annual sediment deposition in Lake Powell is 27,135 ac-ft/yr. The total sediment accumulation in Fall 2005 is estimated to be 1,166,822 ac-ft (1,439,228,264 m3) or 4.3 % of the overall reservoir basin. References Condit, W., Drake, C.L., Mayer, L., Spydell, R., 1978, Sedimentation in Lake Powell in Lake Powell Research Project, no. 64, 88 p. Ferrari, Ron, 1988, 1986 Lake Powell Survey REC-ERC-88-6, USBR, Denver, 67 p.