Laboratory Flume and Numerical Modeling Experiments Show Log Jams and Branching Channels Increase Hyporheic Exchange

Log jams alter gradients in hydraulic head, increase the area available for hyporheic exchange by creating backwater areas, and lead to the formation of multiple channel branches and bars that drive additional exchange. Here, we numerically simulated stream‐groundwater interactions for two constructed flume systems—one without jams and one with a series of three jams—to quantify the effects of interacting jam structures and channel branches on hyporheic exchange at three stream flow rates. In simulations without jams, average hyporheic exchange rates ranged from 2.1 × 10−4 to 2.9 × 10−4 m/s for various stream discharge scenarios, but with jams, exchange rates increased to a range of 1.3 × 10−3–3.5 × 10−3 m/s. Largely due to these increased hyporheic exchange rates, jams increased stream‐groundwater connectivity or decreased the turnover length that stream water travels before interacting with the hyporheic zone, by an order of magnitude, and drove long flow paths that connected multiple jams and channel threads. Decreased turnover lengths corresponded with greater reaction significance per km, a measure of the potential for the hyporheic zone to influence stream water chemistry. For low‐flow conditions, log jams increased reaction significance per km five‐fold, from 0.07 to 0.35. Jams with larger volumes led to longer hyporheic residence times and path lengths that exhibited multiple scales of exchange. Additionally, the longest flow paths connecting multiple jams occurred in the reach with multiple channel branches. These findings suggest that large gains in hydrologic connectivity can be achieved by promoting in‐stream wood accumulation and the natural formation of both jams and branching channels.

原木堵塞改变了水头梯度，通过形成回水区域增加了可供潜流交换的面积，并导致多个河道分支和沙洲的形成，从而推动了额外的交换。在此，我们对两个构建的水槽系统（一个无堵塞，一个有三个连续堵塞）的河流 - 地下水相互作用进行了数值模拟，以量化相互作用的堵塞结构和河道分支在三种河流流速下对潜流交换的影响。在无堵塞的模拟中，对于各种河流流量情景，平均潜流交换速率在2.1×10⁻⁴到2.9×10⁻⁴米/秒之间，但有堵塞时，交换速率增加到1.3×10⁻³ - 3.5×10⁻³米/秒的范围。很大程度上由于这些增加的潜流交换速率，堵塞使河流 - 地下水的连通性增加，或者说使河水在与潜流带相互作用之前所流经的周转长度降低了一个数量级，并形成了连接多个堵塞和河道脉络的长流径。周转长度的降低与每公里更大的反应显著性相对应，反应显著性是衡量潜流带影响河水化学性质潜力的一个指标。在低流量条件下，原木堵塞使每公里的反应显著性提高了五倍，从0.07提高到0.35。体积较大的堵塞导致潜流停留时间更长，流径长度更长，并呈现出多种尺度的交换。此外，连接多个堵塞的最长流径出现在有多个河道分支的河段。这些研究结果表明，通过促进河道内木材堆积以及堵塞和分支河道的自然形成，可以大大提高水文连通性。