Federal dam managers are preparing a springtime assault against smallmouth bass on the Colorado River, possibly using cool water from deep in Lake Powell to keep the non-native fish from getting entrenched in Grand Canyon. Environmental and river recreation advocates hope the U.S. Bureau of Reclamation will pair cold water with a rush of water from deep behind Glen Canyon Dam, both disrupting bass reproduction around Lees Ferry and restoring eroded sandbars farther downstream. That option, officially under study, is politically sensitive because it would cost hydropower production and move water out of a reservoir that has dropped to about a quarter of its storage capacity. The stakes for humpback chub and other native fishes are high. Young smallmouth bass, which grow into voracious warm-water predators, were found between the dam and Lees Ferry last year…