The Role of Headwater Streams in Downstream Water Quality¹

Abstract

Knowledge of headwater influences on the water-quality and flow conditions of downstream waters is essential to water-resource management at all governmental levels; this includes recent court decisions on the jurisdiction of the Federal Clean Water Act (CWA) over upland areas that contribute to larger downstream water bodies. We review current watershed research and use a water-quality model to investigate headwater influences on downstream receiving waters. Our evaluations demonstrate the intrinsic connections of headwaters to landscape processes and downstream waters through their influence on the supply, transport, and fate of water and solutes in watersheds. Hydrological processes in headwater catchments control the recharge of subsurface water stores, flow paths, and residence times of water throughout landscapes. The dynamic coupling of hydrological and biogeochemical processes in upland streams further controls the chemical form, timing, and longitudinal distances of solute transport to downstream waters. We apply the spatially explicit, mass-balance watershed model SPARROW to consider transport and transformations of water and nutrients throughout stream networks in the northeastern United States. We simulate fluxes of nitrogen, a primary nutrient that is a water-quality concern for acidification of streams and lakes and eutrophication of coastal waters, and refine the model structure to include literature observations of nitrogen removal in streams and lakes. We quantify nitrogen transport from headwaters to downstream navigable waters, where headwaters are defined within the model as first-order, perennial streams that include flow and nitrogen contributions from smaller, intermittent and ephemeral streams. We find that first-order headwaters contribute approximately 70% of the mean-annual water volume and 65% of the nitrogen flux in second-order streams. Their contributions to mean water volume and nitrogen flux decline only marginally to about 55% and 40% in fourth- and higher-order rivers that include navigable waters and their tributaries. These results underscore the profound influence that headwater areas have on shaping downstream water quantity and water quality. The results have relevance to water-resource management and regulatory decisions and potentially broaden understanding of the spatial extent of Federal CWA jurisdiction in U.S. waters.

Changing climate and nutrient transfers: Evidence from high temporal resolution concentration-flow dynamics in headwater catchments

“…These first and second order streams make up most of the channel length and basin area of a larger catchment (Burt, 1997), and contribute at least 60-70% of the water (Alexander et al, 2007;Decamps et al, 1999). Many researchers (e.g.…”

Longitudinal and seasonal variation of stream N uptake in an urbanizing watershed: effect of organic matter, stream size, transient storage and debris dams

“…Small headwater streams can be important as well (e.g., Alexander et al 2007) and there is evidence of significant (in some cases more than half of in-stream nitrogen) uptake . Numerous studies have shown that there are multiple controls on in-stream uptake, including both biotic and abiotic factors.…”

“…However, few studies have examined nitrogen processing through a stream channel corridor. Such longitudinal studies are considered essential for understanding the downstream fate of nitrogen (Alexander et al 2007), as well as the associated ecological linkages with organic matter and biological communities (Wipfli et al 2007). Within this spatial context, temporal examinations can further elucidate changes in endogenous and exogenous controls on in-stream nitrogen cycling .…”

Using microbiological tracers to assess the impact of winter land use restrictions on the quality of stream headwaters in a small catchment

“…That hydrological processes in stream headwaters influence the water quality and discharge regimes further downstream has been recognized for some time (Alexander et al, 2007). Although initial studies focused on processes operating in upland catchments where pollution pressures are lower, a large number of rivers and streams across Ireland have headwaters in more intensively cultivated lowland areas with greater water pollution pressures.…”