EXAMINING THE INFLUENCE AND IMPACT OF ANTHROPOGENIC LAND-USE AND CLIMATE-DRIVEN CHANGES IN DISCHARGE ON SMALL WESTERN STREAMS

Anthropogenic land-use and climate change impact rivers and streams throughout the world. In the western United States, urban development, mining, logging, beaver trapping, grazing, and farming affect the hydrology and sediment of fluvial systems. The timing and magnitude of peak stream flow is expected to change under a warming climate; a potential channel response to both land use and climate change is channel incision. Incision is a prevalent and often irreversible process that impacts shallow groundwater resources, aquatic and riparian habitat, and the stability of land and infrastructure.

Lower Dry Creek, a tributary to the Boise River in Idaho, has a history of placer mining, beaver trapping, grazing, and farming since the 1850’s. Recent growth in the region converted Lower Dry Creek’s expansive floodplain from agricultural land to housing developments. In the last ~50 years, a ~1.5 km section of Lower Dry Creek has incised more than five meters below the ‘Hidden Springs’ terrace. It is unknown whether this incision results from changes in land-use or climate-driven changes in discharge. The goals of this project are to 1) examine the timing, magnitude, and spatial extent of incision in Lower Dry Creek; 2) quantify and understand underlying drivers of recent incision in Lower Dry Creek; and 3) apply the results of this study to other similar streams undergoing urbanization.

A geomorphic map correlates low terraces and well-exposed stream deposits to evaluate how Dry Creek incised in the past. Aggradation and incision rates are quantified using ¹⁴C and OSL dating of stream deposits and historical photos. A threshold discharge for incision is estimated using excessive shear stress in both stable and unstable reaches. Influence of climate-driven high flow events are evaluated using the threshold discharge and 20 years of stream flow data. Results of this study have implications of understanding the combined influence of land-use and climate-driven changes in discharge on aquatic resources and working lands and waters of small western streams.