Historic channel dynamics of a highly developed dryland river

Understanding how river systems adjust to hydrologic alterations is critical to sustainable river management. Remote sensing and geographic information systems (GIS) provide a reliable and cost-effective approach to studying river adjustment, including the possible attainment of dynamic equilibrium. We used this approach to study the highly managed lower South Platte River in northeastern Colorado, USA. We assessed the active channel (inclusive of open water, sand bars and adjacent sparsely vegetated areas) in three 30 km study sections, using General Land Office (GLO) maps circa 1870, and a time series of eight sets of aerial photographs from 1941 to 2015. In the GLO surveys, the active channel was 72-80% larger and 65-85% wider than in the later aerial photographs. Mean daily discharge in the prior photo interval was the strongest hydrologic predictor of channel area, while mean June daily discharge was the strongest predictor of channel width. Maximum daily discharge was the only significant predictor of lateral movement of the main channel. The highest rates of channel movement occurred when the primary channel switched locations to occupy a pre-existing secondary channel. An average of 17% of the total active channel area was persistent, defined as consisting of channels at >= 7 of the eight photo years, and there was a narrow, persistent active channel throughout 68-72% of the length of each section. Thus, the location of the river has been stable since 1941, though flanking the channel location were areas of the active channel that were more temporally dynamic. After 1956, the post-development lower South Platte River channel planform achieved a state of dynamic equilibrium, fluctuating thereafter within a relatively narrow range of values. As conditions in the basin change in the future due to increased municipal and industrial water demand and climate change, we can expect the river to also change. Historical maps, aerial imagery and GIS were used to assess river planform adjustment of the highly managed South Platte River in Colorado, USA. The active channel narrowed by 70-80% between 1870 and 1941, and then remained fixed in position, as indicated by the existence of a narrow, persistent channel between 1941 and 2015. River planform is responsive to changes in driving variables, namely streamflow, exhibiting dynamic equilibrium in channel area and width since 1956. image