Water quality effects of stream restoration in the Chesapeake Bay watershed: benefits and unintended consequences
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Stream restoration is a popular but expensive approach for managing nutrients (nitrogen and phosphorus) and sediment dynamics in urban watersheds. However, questions about costs and benefits remain. Here, we discuss lessons learned over 20 years of investigating effects of geomorphic stream restoration on riparian and in-stream nitrogen transport and transformation in urban streams in the Chesapeake Bay watershed. We examined relationships between hydrology, chemistry, and biology to determine how flashiness, and nutrient concentrations and flux, changed after restoration. We present data from multiple sites and various restoration approaches including natural channel design, regenerative stormwater conveyance, and removal of legacy sediments. We show that restoration can be an effective nutrient management approach dependent upon re-establishing groundwater-surface water interaction, addressing erosion and shear stress, protecting riparian zones, and ensuring sufficient cycling of organic matter for microbial activity. Benefits observed include: 1) reduced concentration and loads of nitrogen, 2) reduced peak flows, flashiness, and shear stress, and 3) increased denitrification. There are also potential unintended negative consequences including: 1) mobilization of metals and ions after tree removal, 2) erosion and channel degradation from poor channel design and underestimation of peak flows, and 3) low dissolved oxygen from reduced flows. We identify remaining data gaps, critical questions, and monitoring needs for elucidating effective nutrient BMPs.