We studied the 1941 diversion of Furnace Creek Wash (drainage area 439 km2) into Gower Gulch (5.8 km2) as an experiment in the transient response of a channel to a large change in water and sediment discharge. We measured sequential changes in valley width using a time series of aerial photographs (1948–1995), airborne laser elevation data from 2005, and a field survey. We found that the response of the system varied depending on the prediversion channel morphology and bedrock geology. In two steep knickzone segments, narrowing, knickpoint retreat and bedrock incision dominated, indicating a detachment-limited response. In the relatively low-gradient main part of Gower Gulch, widening dominated as the coarse postdiversion sediment load covered the channel bed. This transport-limited part of the system has undergone only modest incision and adjustments in gradient. Over long periods, the lowering rate of Gower Gulch probably depends on knickpoint retreat, but the present-day response of this non–steady-state system is a hybrid of incision and narrowing in detachment-limited reaches and widening in transport-limited reaches. This system demonstrates the importance of evolving channel geometry in setting the transient response of rivers to changes in forcing parameters.