Monitoring a Non-Perennial Stream in a Semi-Arid Rangeland Slated for Development

Urbanization has recognizable impacts on stream morphology and flow patterns. However, accurately predicting and quantifying these impacts remains difficult. This is especially true for non-perennial streams in semi-arid rangelands. Non-perennial streams tend to have an incomplete record for streamflow presence or absence. Lacking a historical baseline can make accurately predicting and planning for post-development stream changes difficult. This study focuses on a non-perennial stream within West Stroh Gulch, a semi-arid rangeland undergoing housing development south of Denver, Colorado, USA. Stream presence and absence is recorded at three locations along the stream network with time-lapse photography. Recorded precipitation events and corresponding photos are reviewed to quantify what storm intensities and cumulative depths do, or do not, trigger a flow response. The majority of pre-development observations have indicated a dry stream channel. Out of 772 analyzed days, there were 124 recorded precipitation events. Soil moisture was observed for 102 out of those 772 days. Three of the days with observed soil moisture also captured images of standing water in the channel. In addition to continuing time-lapse photography observations, a HEC-RAS model of the watershed will be built using topographic data collected from drone flyovers. SWMM generated post-development flows will be entered into the HEC-RAS model to predict post-development changes to the channel morphology. Combining model predictions and ongoing observations will help create a valuable case study of pre to post development changes to a non-perennial stream in a semi-arid rangeland.