A study of the riverine-estuarine reach of the Knik and Matanuska Rivers provided flow and hydraulic data for use in the design of additional bridges over the rivers. Hydraulic analysis is complicated because: (1) the lower reaches of the rivers merge in a complex system of interconnected channels; and (2) this reach is subject to unsteady flow conditions resulting from a semidiurnal tide wave propagated up the channel through Knik Arm from Cook Inlet, whose tidal range is among the largest in the world. Analysis of flows for the Knik River is further complicated by the historic formation and outburst flooding of glacier-dammed Lake George in the Upper Knik River basin. Peak flows on the Knik River due to breakout floods were as much as seven times greater than peak flows of non-breakout floods. The U.S. Geological Survey 's branch-network flow model was used to simulate flows within the study reach. For the Knik River, simulated flows were within 10% of measured values in most cases. The model was also used to simulate the flow, stage, and velocity that would be expected in the various channels under different bridge configurations.
