Internal structure of bars in Grand Canyon, Arizona, and evaluation of proposed flow alternatives for Glen Canyon Dam

Abstract

Channel expansions are sites of deposition in bedrock canyons and alluvial rivers. Within these areas, deposition is commonly focused at the separation point, reattachment point, eddy center, or along the shear surface that separates the recirculation zone from the downstream flow in the main channel. This study examined the internal structure of reattachment bars, separation bars, and natural levees along the Colorado River in Grand Canyon, Arizona. The recirculation deposits (separation and reattachment bars) are characterized by a rotary flow pattern that includes upstream flow. Flow patterns within channel expansions vary with discharge. Increases in discharge generally increase the length of the recirculation zone. Both the rotary flow pattern and lengthening of the recirculation zone have been documented from internal structures in the bars. The character of climbing-ripple structures in the bar deposits demonstrates that recirculating flows pulsate erratically; field and lab current-meter measurements have demonstrated that these erratic pulsations occur even when discharge in the main channel is steady.