Fire and ice: Winter flooding in a Southern Rocky Mountain stream after a wildfire

Abstract

We observed a low-discharge flooding phenomenon on Little Beaver Creek, a tributary to the Cache la Poudre River in the Southern Rocky Mountains of Colorado, USA. Ice ranging in thickness from a few centimeters to 0.5 m occupied a large volume of the channel, forcing flows out of its banks. On two occasions, multiple consecutive days of uncharacteristically warm weather caused a spike in snowmelt, and bankfull stage was surpassed 60 days before peak runoff. The flow events occurred in late winter, six months after a wildfire had burned a large portion of the watershed at medium-to-high severity. Ash and other organic materials were mobilized during the overbank flows, and we observed evidence of recent ash deposition in the floodplain and high ash concentrations in the channel. During the same time period, camera footage captured the formation and collapse of a large accumulation of ice and snow at a log jam. Because climate change projections and recent observations indicate higher variability in weather patterns, wildfire regime, and precipitation events, winter flooding associated with ice, although not widely documented in small mountain streams, may become more common. Flooding and subsequent deposition of ash and other sediment can disrupt fluvial processes, impair the local biotic community, cause property damage, and impact drinking water sources.