The Influence of Forested headwater Stream Hydrology on Target TMDLs

Abstract

Headwater streams are often intermittent. In the dry summer season these streams are reduced to pools with no surface flow, or even to completely dry beds. During the wet winter season, higher stream levels inundate backwater areas and drainage beds adjacent to the main channel. Understanding the headwater stream hydrology is important to evaluate how forest management practices can potentially affect downstream water quality. This study was conducted in the Flat Creek Watershed, a 350 km2 drainage area in North Central Louisiana, a region dominated by commercial pine forests and pasture. The streams in this watershed have highly variable flow not only due to seasonality, but also because of beaver dams and other log or debris buildups that create pools of stagnation directly upstream. In this study, streamflow, total suspended solids (TSS), and total dissolved solids (TDS) were monitored at a first and second order stream from December 2005 to November 2006. Streamflow was also simulated using the Soil and Water Assessment Tool (SWAT) to estimate flow at previously ungaged locations. The results show that due to the first and second order nature of these streams, response to precipitation occurs quickly, even with the relatively flat topography. The combination of distinct seasonality, low slopes, and rapid storm response produces a large variability in the level of sediment concentrations over time and within the watershed. Monthly TSS loadings averaged 9,223 kg at the first order stream and 4,786 kg at the second order stream. Monthly TDS loadings averaged 3,707 kg at the first order and 40,092 kg at the second order stream. Differences in sediment loading with hydrologic variability in a watershed can lead to large ranges in water quality, and suggests that local stream hydrologic conditions need to be considered when determining target stream loadings for TMDL requirements.