Abstract:Confluences are important locations for river mixing within drainage networks, yet few studies have examined in detail the dynamics of mixing within confluences. This study examines the influence of momentum flux ratio, the scale of the flow (crosssectional area) and the density differences between incoming flows on thermal mixing at a small stream confluence. Results reveal that rates and patterns of thermal mixing depend on event-specific combinations of the three factors. The mixing interface at this confluence is generally distorted towards the mouth of the lateral tributary by strong helical motion associated with curvature of flow from the lateral tributary as it aligns with the downstream channel. As the momentum flux from the lateral tributary increases, mixing is enhanced because helical motion from the curving tributary flow expands over the width of the downstream channel. The cross-sectional area of the flow is negatively correlated with mixing rates, suggesting that the amount of mixing over a fixed distance downstream of the confluence is inversely related to the scale of the flow. Density differences are not strongly related to rates of mixing. Results confirm that mixing rates within the region of confluent flow interaction can be highly variable among flow events with different incoming flow conditions, but that, in general, length scales of mixing are short, and rates of mixing are high at this small confluence compared with those typically documented at large-river confluences.

Large-scale particle image velocimetry (LSPIV) has emerged as a valuable tool for measuring surface velocity in a variety of fluvial systems. LSPIV has typically been used in the field to obtain velocity or discharge measurements in relatively simple one-dimensional flow. Detailed two-dimensional or threedimensional characterization of flow structure has been relegated to laboratory settings because of the difficulty in controlling PIV limiting factors such as poor particle seeding, the need for camera rectification, and challenging field conditions. In this study we implement a low-cost LSPIV setup using a high-resolution action camera mounted above a stream confluence and water seeded with recycled landscape mulch. Time-averaged 2-D velocities derived from LSPIV are compared with those measured with an acoustic Doppler velocimeter (ADV) in the camera's field of view. We also assess the capabilities of this setup to resolve turbulent and time-averaged flow structures at a stream confluence. Our results reveal that even in challenging field conditions a basic LSPIV setup can yield accurate data on velocity and resolve in detail the temporal evolution of flow structures on the surface of rivers. The resulting dataset contains velocity information at high spatial and temporal resolution, a significant advance in understanding flow processes at stream confluences. Our LSPIV analysis provides support for previous numerical modeling studies that have distinguished between Kelvin-Helmholtz and wake modes of turbulent behavior within the mixing interface at confluences. This study shows that LSPIV can provide unprecedented levels of resolution of surface velocity patterns on rivers. Detailed velocity data derived from LSPIV can be used to evaluate numerical predictions of flow structure in complex fluvial environments.

Confluences are locations of complex hydrodynamic conditions within river systems. The effects on hydrodynamics and mixing of temperature-induced density differences between incoming flows are investigated at a small-size, concordant bed confluence. To evaluate density effects, results of eddy-resolving simulations for a densimetric Froude number Fr = 4.9 (weak-density-effects cases) and Fr = 1.6 (strong-density-effects cases) are compared to results of simulations in which the densities of the incoming flows do not differ (no-density-effects cases). Flow patterns predicted for both weak-and strong-density-effects cases show that secondary flow develops with increasing distance from the confluence apex. The pattern of secondary flow is characterized by denser fluid on one side of the confluence moving near the bed toward the side of the downstream channel corresponding to the less dense fluid and the less dense fluid moving near the free surface in the opposite direction. This pattern of fluid motion is similar to a spatially evolving lock-exchange cross flow. In the strong-density-effects simulations, a cross-stream cell of secondary flow develops at the density interface between the flows, similar to interfacial billows generated in classical lock-exchange flows. Density effects increase global mixing with respect to corresponding no-density-effects cases regardless of whether the high-momentum stream contains the higher-density fluid or the lower-density fluid. When density effects are weak, the lock-exchange mechanism either reinforces the pattern of mixing associated with secondary flow induced by inertial forces, particularly helical motion, or opposes this pattern of mixing, depending on which tributary contains the denser fluid. When density effects are strong, flow from the upstream channel with the denser fluid moves under the flow from the upstream channel with the less dense fluid.

Humans have become major geomorphological agents, effecting substantial change in the characteristics of Earth's physical landscapes. The agricultural Midwest of the United States is a region marked by pronounced human influence at the landscape scale. Humans undoubtedly have strongly influenced critical zone processes, including fluvial processes, in intensively managed agricultural landscapes, yet the exact nature of human alteration of these processes is unknown. This study documents historical changes in the extent of the stream channel network and in channel planform within the upper Sangamon River basin -an intensively managed agricultural watershed in Illinois. Results indicate that the modern channel network is nearly three times more extensive than the channel network in the 1820s. Most change in drainage density has occurred in headwater portions of the basin where numerous drainage ditches have been added to the network to drain flat uplands. No detectable change in channel position is evident between 1940 and 2012 along about 60% of the total length of the Sangamon River and its major tributaries. Nearly 30% of the total length exhibits change related to meander dynamics (cutoffs and lateral migration), whereas about 8% has changed as a result of channelization. Channelized sections typically remain straight for decades following human modification, supporting the notion that humans produce long-lasting catastrophic change in channel planform in this region. The findings confirm that humans are effective agents of morphological change in fluvial systems in this intensively managed watershed. Documenting human-induced versus natural changes in fluvial systems is important for evaluating how other critical zone processes in intensively managed landscapes have been affected by these changes. Humaninduced changes in channel extent and planform most likely have altered this landscape from one dominated by biogeochemical transformations and storage of water and sediment prior to agricultural development into one now characterized by enhanced fluxes of water, sediment, and nutrients.

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