Mapping increases in hyporheic exchange from channel-spanning logjams

Human impacts such as timber harvesting, channel engineering, beaver removal, and urbanization alter the physical and chemical characteristics of streams. These anthropogenic changes have reduced the number of fallen trees and amount of loose wood that form blockages in streams. Logjams increase hydraulic resistance and create hydraulic head gradients along the streambed that drive groundwater-surface water exchange. Here, we quantify changes in hyporheic exchange flow (HEF) due to a channel-spanning logjam using field measurements and numerical modeling in MODFLOW and MT3DMS. Electrical resistivity (ER) imaging was used to monitor the transport of solutes into the hyporheic zone during a series of in-stream tracer tests supplemented by in-stream monitoring. We conducted experiments in two reaches in Lithe Beaver Creek, Colorado (USA): one with a single, channel-spanning logjam and the second at a control reach with no logjams. Our results show that 1) higher HEF occurred at the reach with a logjam, 2) logjams created complex HEF pathways that caused bimodal solute breakthrough behavior downstream, and 3) higher discharge rates associated with spring snowmelt increased the extent and magnitude of HEF. Our numerical modeling supported all three field findings, and also suggested that lower flows increased solute retention in streams, although this last conclusion was not supported by field results. This study represents the first use of ER to explore HEF around a naturally occurring logjam over different stream discharges, and has implications for understanding how logjams influence the transport of solutes, the health of stream ecosystems, and stream restoration and conservation efforts.

诸如木材采伐、河道工程、海狸移除以及城市化等人类活动影响改变了河流的物理和化学特性。这些人为变化减少了在河流中形成堵塞的倒伏树木数量和松散木材量。原木堵塞增加了水流阻力，并在河床沿线产生水头梯度，从而驱动地下水 - 地表水交换。在此，我们利用现场测量以及在MODFLOW和MT3DMS中的数值模拟，对因横跨河道的原木堵塞而导致的潜流交换流（HEF）变化进行量化。在一系列河流示踪剂测试（同时辅以河流内监测）期间，使用电阻率（ER）成像来监测溶质向潜流带的输送。我们在美国科罗拉多州利思比弗溪的两个河段进行了实验：一个河段有一处横跨河道的原木堵塞，另一个作为对照河段没有原木堵塞。我们的研究结果表明：1）有原木堵塞的河段出现了更高的HEF；2）原木堵塞创造了复杂的HEF路径，导致下游溶质突破呈现双峰行为；3）与春季融雪相关的更高流量增加了HEF的范围和强度。我们的数值模拟支持所有三项现场发现，并且还表明较低的流量增加了河流中溶质的滞留，尽管这最后一个结论没有得到现场结果的支持。这项研究首次利用ER来探索在不同河流流量下自然形成的原木堵塞周围的HEF，并且对理解原木堵塞如何影响溶质运输、河流生态系统健康以及河流修复和保护工作具有重要意义。