Tranmer Andrew W, Bertagnoli Andrea, Hurst Aaron, Ubing Caroline, Sholtes Joel, Tonina Daniele
Center for Ecohydraulics Research, Department of Civil and Environmental Engineering, University of Idaho, 322 E. Front St., Boise, ID 83702, USA.
Center for Ecohydraulics Research, Department of Civil and Environmental Engineering, University of Idaho, 322 E. Front St., Boise, ID 83702, USA.
Sci Total Environ. 2025 Jan 1;958:177890. doi: 10.1016/j.scitotenv.2024.177890. Epub 2024 Dec 12.
Water temperature is a fundamental driver of physical processes, metabolic rates, and habitat availability in fluvial systems. As anthropogenic activities and climate change increase river temperatures and associated thermal stress on aquatic organisms, river restoration has focused on moderating thermal regimes and creating localized cold-water refuges. Restoration of a 2.5 km-long gravel-bed reach of the Grande Ronde River (Oregon, USA) provided a unique opportunity to compare the pre- and post-restoration influences of hyporheic and groundwater discharge and pool stratification in generating local and reach-scale thermal buffering under similar valley and riparian conditions. Field data and numerical simulations demonstrate that hyporheic discharge had limited influence on the surface water temperature because of its low magnitude relative to the streamflow. In contrast, pools excavated during restoration, with flow depths up to 2.2 m, showed thermal stratification during summer low-flow conditions that generated a temperature differential in the pool bottoms up to 4.6 °C. The duration that an individual pool could maintain water temperatures below the mortality threshold for salmonids (T < 25 °C) increased with dimensionless residual pool depth (D = D/D, where D is the residual pool depth and D the mean flow depth). However, no pools were capable of maintaining sublethal temperatures for the entire summer period. The cumulative effect of stratified pools generated reach-scale thermal buffering by exporting cool water downstream during the day. Measured field values indicate that relative to the upstream reach boundary, maximum daily water temperatures at the downstream boundary of the study site would increase by an average of 1.5 °C before restoration but decrease by an average of 3 °C after restoration. At the reach scale, a sequence of stratified pools induced downstream thermal buffering but could not completely relieve the unsuitable warm water temperatures entering the upstream end of the reach.
水温是河流系统中物理过程、代谢率和栖息地可利用性的基本驱动因素。随着人为活动和气候变化导致河流水温升高以及对水生生物的相关热应激增加，河流修复工作聚焦于调节热状况并创建局部冷水避难所。对美国俄勒冈州格兰德朗德河一段2.5公里长的砾石床河段进行修复，提供了一个独特的机会，可在相似的山谷和河岸条件下，比较修复前后潜流和地下水排放以及水潭分层在产生局部和河段尺度热缓冲方面的影响。实地数据和数值模拟表明，由于潜流排放相对于溪流流量较小，其对地表水温度的影响有限。相比之下，修复过程中挖掘的水潭，水流深度达2.2米，在夏季低流量条件下显示出热分层现象，水潭底部产生的温差高达4.6°C。单个水潭能够将水温维持在鲑鱼致死阈值以下（T < 25°C）的持续时间，随无量纲剩余水潭深度（D = D/D，其中D为剩余水潭深度，D为平均水流深度）增加。然而，没有水潭能够在整个夏季维持亚致死温度。分层水潭的累积效应通过在白天向下游输出冷水，产生了河段尺度的热缓冲。实测的实地数据表明，相对于上游河段边界，研究地点下游边界的最高日水温在修复前平均会升高1.5°C，但在修复后平均会降低3°C。在河段尺度上，一系列分层水潭引发了下游热缓冲，但无法完全缓解进入河段上游端的不适宜温水温度。
