GEOMORPHIC CHANGE AND BIOGEOMORPHIC FEEDBACKS IN A DRYLAND RIVER: THE LITTLE COLORADO RIVER, ARIZONA, USA

The Little Colorado River (LCR) in northern Arizona, has undergone substantial geomorphic change. We analyzed instantaneous discharge data starting in the 1920s to quantify changes in total flow and peak flow. We also analyzed aerial photographs, discharge and sediment-transport measurements, and repeated historical cross sections to determine the rate and magnitude of geomorphic change.

Streamflow analyses show that there have been 4 alternating periods of high and low total flow; 1926 to 1943 (high), 1944-1964 (low), 1965 to 1995 (high), and 1995 to present (low). Previous research showed that fluctuations in total flow were primarily driven by variations in climate. Irrespective of these variations in total flow, peak flow has progressively declined with substantial associated geomorphic change. Importantly, similar progressive declines in peak flow have not occurred in adjacent rivers with smaller alluvial valleys and less water development.

Since 1936, the channel in alluvial valleys of the LCR has narrowed by up to 88%. Channel narrowing occurred through lateral and vertical accretion of sediment on the channel margin and floodplains, with associated vegetation encroachment. In some reaches, the channel-bed aggraded by up to ~1.5 m. Since the 1930s, there have been only 2 floods that caused channel widening; in 1972/73 and 1993. Channel narrowing resumed shortly after each of these floods, such that these floods had minimal long-term effect on channel geometry.

Although the 1940s decline in peak flow can be attributed to climatic variation, the continued decline in peak flow despite the rebound of wetter conditions between 1965-1995 indicates the importance of other forcing mechanisms: (1) large scale water development decreasing the occurrence of floods; (2) vegetation encroachment on the channel margins and floodplains, resulting in increased roughness, sediment trapping, channel narrowing, and peak-flow attenuation; and (3) the increase in channel length by approximately 34-48% caused by an increase in sinuosity and biogeomophic feedbacks resulting in additional peak-flow attenuation. Thus, water development and the biogeomorphic feedbacks that followed the initial 1940s decline in peak flow have likely been the main drivers of the post-1940s progressive decline in peak flow in the LCR.