Transverse Bars and Braiding in the Lower Platte River, Nebraska

Abstract

The Platte is a wide, shallow river which flows eastward from the Rocky Mountains across the Great Plains of Nebraska. Its lower reaches carry a dominantly sandy load and during intermediate and low discharges display a pronounced braided character accomplished primarily through dissection of tabular, flat-topped transverse bars. Transverse bars form by sediment aggrading to a profile of equilibrium (Jopling, 1966) and grow by downcurrent extensions of avalanche faces. Depth, velocity, and grain size tend to decrease on active bar surfaces from their upstream mouths to the downstream and lateral margins. Active surfaces are covered with small-scale bed forms whose distributions are controlled by the flow characteristics. A typical mouth-to-margin bed form progression is dunes to diminished dunes to ripples, reflecting downcurrent reduction of stream power. Water-surface slopes over active bars tend to be greater than those of the channel segments which feed them. Under ideal conditions, transverse bars are essentially lobate; however, most bars, especially during low discharges, assume irregular or asymmetrical patterns due to any of several factors that include bar-mouth cross-sectional geometry, proximity to exposed banks, adjacent currents, steadiness of flow, and basin depth distribution. Braiding (bar dissection) begins during decreasing discharges when the flow passing through the bar mouth becomes unable to sustain active sediment transport over the entire bar surface. A single bar, examined closely over a five-day period of gradually decreasing discharge, documents the evolution from wholly active to dissected states.