Ground-water conditions in the Lake Powell area, Utah

Loss Rates from Lake Powell and Their Impact on Management of the Colorado River

“…His groundwater model showed that within 20 years, groundwater levels will have increased 100 m near the downstream end of the reservoir and more than 7 m, up to 40 km from the Colorado River. However, the potentiometric surface would still slope toward the river, as found by Blanchard (). Neither simulations nor observations suggest a groundwater divide has or will form to prevent water from returning to the reservoir; a divide could form downgradient of the dam causing water to return to the river far below the dam (Thomas, ).…”

“…The unsaturated thickness from the original river level to an approximate average depth is about 152 m at the dam (Thomas, ). The pre‐reservoir groundwater level sloped up from the river to a point where the groundwater level approximates the full reservoir elevation, about 8 km from the river at the dam (Blanchard, ). The volume of an unsaturated wedge in the sandstone surrounding the reservoir is about 180 Gm 3 accounting for the riverbed slope.…”

“…Groundwater flowing toward the river likely no longer reaches it due to backwater caused by the rising reservoir because of backwater into the sediments above the reservoir level (Blanchard, ; Thomas, ), as seen in rising groundwater levels near the reservoir (Figures and ). Thomas' () groundwater model simulations found that a long‐term equilibrium would result in about 400 years with half of the total bank storage having occurred by 1983.…”

“…The Colorado Plateau consists of nearly horizontal sedimentary strata deeply incised by major stream systems and interrupted by north‐south trending monoclines, structural domes, and basins along with widely scattered extrusive and intrusive igneous features (Blanchard, ). The Navajo Sandstone forms the walls of Glen Canyon, and contains and transmits any bank storage and seepage.…”

“…Local inflow from this semiarid area includes groundwater discharge, local runoff, and several perennial rivers. Groundwater inflow would be relatively constant to the river or to the reservoir basin although development of the reservoir would change the hydraulic gradient so that the discharge to the reservoir would be lessened or even reversed (Blanchard, ; Thomas, ). Local runoff and stream inflow would continue unchanged due to the reservoir except for a portion of the area covered by the reservoir.…”