Irradiation of particulate organic matter (POM) at light intensities found at the earth's surface should induce reduction in molecular weight, as found for dissolved organic matter, and hence result in transfer to the dissolved phase. We studied Mississippi River suspended sediments to test if photodissolution can induce losses of POM similar to those observed between delivery and burial in coastal sediments. Irradiation experiments in a solar simulator demonstrated dissolution of tens of percent of the POM after several days of exposure to strong sunlight. Neither water type nor iron oxyhydroxide removal had large effect on the reaction extent, but temperature may be a strong controlling parameter. Ultraviolet and visible wavelengths drive this reaction. A hyperbolic response of reaction extent to photon flux allows significant reaction to occur in highly turbid suspensions, despite significant light penetration into the suspensions of only millimeters to centimeters. Our data do not yet allow quantitation of this reaction's contribution to POM loss between the Mississippi River and its depocenter, but they do demonstrate its potential significance under nearshore resuspension regimes. More importantly, these results point to a heretofore ignored role for photodissolution of particulate organic matter at the earth's surface.

Feedbacks between woody plants and fluvial morphodynamics result in co-development of riparian vegetation communities and channel form. To advance mechanistic knowledge regarding these interactions, we measured the response of topography and flow to the presence of riparian tree seedlings with contrasting morphologies in an experimental, field-scale, meandering stream channel with a mobile sand bed. On a convex point bar, we installed seedlings of Tamarix spp. (tamarisk) and Populus fremontii (cottonwood) with intact roots and simulated a bankfull flood, with each of eight runs varying sediment supply, plant density, and plant species. Vegetation reduced turbulence and velocities on the bar relative to bare-bed conditions, inducing sediment deposition when vegetation was present, regardless of vegetation density or species. Sediment supply also played a dominant role, and eliminating sediment supply reduced deposition regardless of the presence of vegetation. Unexpectedly, plant density and species architecture (shrubby tamarisk vs. single-stemmed cottonwood) had only a secondary influence on hydraulics and sediment transport. In the absence of plants, mobile bedforms were prominent across the bar, but vegetation of all types decreased the height and lateral extent of bedforms migrating across the bar, suggesting a mechanism by which vegetation modulates feedbacks among sediment transport, topography, and hydraulics. Our measurements and resulting insights bridge the gap between laboratory conditions and real dryland sand-bed rivers and motivate further morphodynamic modeling.

Understanding fluvial adjustments to base level changes benefits the fields of sequence stratigraphy, geomorphology and petroleum geology. This investigation is a modern case study of the channel dynamics of Lee Creek and the Goggin Drain, two streams that are part of the Jordan River drainage into the endorheic Great Salt Lake of northern Utah, a lacustrine system that has experienced multiple, decadal-scale base level changes. Since 1965, the lake level has fluctuated in elevation more than 6 m, transitioning from an historic lowstand [< 1279 m above sea level (a.s.l.)] to an historic highstand (>1284 m a.s.l.), and in 2009-2010 approaching an historic lowstand. This study uses detailed aerial images, fieldwork and LiDAR data to link the modern geomorphology and channel hydraulics to specific variations in sediment transport, channel form, and avulsion behavior. Although Lee Creek and the Goggin Drain are situated only a few kilometers apart and share similar shore zone gradients, substrates and vegetation patterns, and have been subjected to the same changes in lake level, their channel forms have evolved very differently. Differences in discharge patterns are likely the most influential factor causing the meandering form of Lee Creek and the braiding channel of the Goggin Drain. Despite the differences in discharge, total sediment eroded from the two streams is comparable and can be attributed to similar stream power/unit stream width in the two streams. Although Lee Creek has not recently been avulsive, three major avulsions of the Goggin Drain have taken place since 1965. Two possible styles of avulsion are interpreted: an allogenic response to changing base level, and an autogenic response dictated by channel morphology and hydraulics. Despite a wealth of available information, avulsions cannot be unequivocally attributed to one style or another. Caution should be used when attempting to link the complex process of avulsion to causal mechanisms.

Streamflow depletion is occurring globally, due to land use change, climate change, and increasing human water demand. Ecological effects of low flows are particularly significant for diadromous fish, which require connected stream networks to migrate between fresh and marine waters. In coastal California, USA, drying streams are known to limit rearing habitat for juvenile salmon, but effects on their seaward migration remain poorly understood. In this study, we evaluated the outmigration of endangered, juvenile coho salmon ( Oncorhynchus kisutch) during the late spring flow recession in four streams over 10 years. We monitored the outmigration of fish tagged with passive integrated transponders via detections at stationary antennas, and we measured stream water depths when movement was detected. We assessed depths at multiple riffle crest thalwegs (RCTs), the shallowest geomorphic feature that fish must navigate. Finally, we calculated population-level outmigration depth preferences by evaluating depths during fish movement, relative to depths available during the potential outmigration window. Juvenile fish moved over a wide range of depths (interquartile range 6.1–18.0 cm), which varied by year and stream. Fish ceased to move at shallow water depths, which limited late-season outmigration as stream drying occurred. Our findings suggest that management actions to increase streamflow during the spring would benefit salmon outmigration and could contribute to population recovery. Streamflow-RCT depth relationships, used to assess coho depth preferences during movement, is a relatively simple and effective method for assessing environmental flow needs, a priority for aquatic conservation in California and globally.