Stream restoration strategies for reducing river nitrogen loads

Abstract

Despite decades of work on implementing best management practices to reduce the movement of excess nitrogen (N) to aquatic ecosystems, the amount of N in streams and rivers remains high in many watersheds. Stream restoration has become increasingly popular, yet efforts to quantify N‐removal benefits are only just beginning. Natural resource managers are asking scientists to provide advice for reducing the downstream flux of N. Here, we propose a framework for prioritizing restoration sites that involves identifying where potential N loads are large due to sizeable sources and efficient delivery to streams, and when the majority of N is exported. Small streams (1st–3rd order) with considerable loads delivered during low to moderate flows offer the greatest opportunities for N removal. We suggest approaches that increase in‐stream carbon availability, contact between the water and benthos, and connections between streams and adjacent terrestrial environments. Because of uncertainties concerning the magnitude of N reduction possible, potential approaches should be tested in various landscape contexts; until more is known, stream restoration alone is not appropriate for compensatory mitigation and should be seen as complementary to land‐based best management practices.

Phosphorus and Nitrogen Dynamics in Riverine Systems: Human Impacts and Management Options

“…Diffuse inputs from agricultural land use include agricultural fertilizers and increased soil leachate and erosion due to tillage. As SRP and ammonium easily adsorb to charged soil particles, these nutrients enter streams and rivers mainly via soil erosion (Craig et al 2008;Withers and Jarvie 2008) (Figs. 10.1 and 10.3).…”

“…Due to their diverse channel morphology and their low discharge, pristine headwaters can retain large amounts of nutrients via benthic uptake, thereby controlling nutrient transport into downstream reaches (Craig et al 2008). Stream regulation due to urbanization or agricultural land use results in a homogenization of the stream channel and an acceleration of water flow and, thus, decreases the physical retention function of headwater streams (Ensign and Doyle 2005).…”

“…Channel reconfiguration, such as channel widening and remeandering, and the restoration of structural complexity via the addition of flow obstructions (e.g., debris dams, side pools, and diversification of bed materials) may enhance nutrient uptake by increasing water residence time, promoting contact between the water and the sediment surface, and enhancing the hyporheic water exchange (Bukaveckas 2007;Craig et al 2008;Hines and Hershey 2011). Woody material on the stream bed additionally increases the nutrient demand of the decomposing microorganisms due to the high C-N and C-P ratios (Roberts et al 2007).…”

“…Woody material on the stream bed additionally increases the nutrient demand of the decomposing microorganisms due to the high C-N and C-P ratios (Roberts et al 2007). Besides, debris dams provide organic carbon for in-stream denitrification (Craig et al 2008). For example, the creation of riffles, cross vanes, and step pools within a restored stream shortened NH 4 -N uptake lengths from 200 to 70 m (Hines and Hershey 2011).…”

Effects of inset floodplains and hyporheic exchange induced by in-stream structures on nitrate removal in a headwater stream

“…Many stream restoration practices are considered for mitigating water quality impacts, including channel realignment, riparian planting, in-stream structure installation, and floodplain reconnection (Roni et al, 2002;Ensign and Doyle, 2005;Kaushal et al, 2008;Opperman et al, 2009;Hester and Gooseff, 2010;Mason et al, 2012;Azinheira et al, 2014;Johnson et al, 2015;Jones et al, 2015). Yet water quality improvement is a relatively new goal compared to more traditional objectives such as bank stabilization, ecosystem enhancement or riparian zone management (Bernhardt et al, 2005), and little guidance is available to guide stream restoration design for purposes of improving N removal from the channel (Craig et al, 2008;Veraart et al, 2014;Johnson et al, 2015).…”