The North River estuary (Massachusetts, USA) is a tidal marsh creek network where tidal dispersion processes dominate the salt balance. A field study using moorings, shipboard measurements, and drone surveys was conducted to characterize and quantify tidal trapping due to tributary creeks. During flood tide, saltwater propagates up the main channel and gets “trapped” in the creeks. The creeks inherit an axial salinity gradient from the time-varying salinity at their boundary with the main channel, but it is stronger than the salinity gradient of the main channel because of relatively weaker currents. The stronger salinity gradient drives a baroclinic circulation that stratifies the creeks, while the main channel remains well-mixed. Because of the creeks’ shorter geometries, tidal currents in the creeks lead those in the main channel; therefore, the creeks never fill with the saltiest water which passes the main channel junction. This velocity phase difference is enhanced by the exchange flow in the creeks, which fast-tracks the fresher surface layer in the creeks back to the main channel. Through ebb tide, the relatively fresh creek outflows introduce a negative salinity anomaly into the main channel, where it is advected downstream by the tide. Using high-resolution measurements, we empirically determine the salinity anomaly in the main channel resulting from its exchange with the creeks to calculate a dispersion rate due to trapping. Our dispersion rate is larger than theoretical estimates that neglect the exchange flow in the creeks. Trapping contributes more than half the landward salt flux in this region.

北河河口（美国马萨诸塞州）是一个潮汐沼泽小溪网络，潮汐扩散过程控制着盐分平衡。使用系泊、船上测量和无人机调查进行了实地研究，以表征和量化由于支流小溪引起的潮汐诱捕。在涨潮期间，咸水沿主河道向上传播并“困”在小溪中。小溪从其与主河道边界处的时变盐度继承了轴向盐度梯度，但由于相对较弱的水流，它比主河道的盐度梯度强。较强的盐度梯度推动了使小溪分层的斜压环流，而主河道保持良好混合。由于小溪的几何形状较短，小溪中的潮汐领先于主河道中的潮汐；所以，小溪永远不会充满通过主要通道交界处的最咸的水。小溪中的交换流增强了这种速度相位差，从而使小溪中较新鲜的表层快速返回主渠道。通过退潮，相对新鲜的小溪流出将负盐度异常引入主河道，在那里它被潮汐平流向下游。使用高分辨率测量，我们凭经验确定主通道中由于与小溪交换而导致的盐度异常，以计算由于捕获而导致的分散率。我们的分散率大于忽略小溪中交换流量的理论估计值。在该地区，捕集贡献了一半以上的陆上盐通量。