The lower Green River episodically narrowed between the mid-1930s and present day through deposition of new floodplains within a wider channel that had been established and/or maintained during the early twentieth century pluvial period. Comparison of air photos spanning a 74-yr period (1940−2014) and covering a 61 km study area shows that the channel narrowed by 12% from 138 ± 3.4 m to 122 ± 2.1 m. Stratigraphic and sedimentologic analysis and tree ring dating of a floodplain trench corroborates the air photo analysis and suggests that the initial phase of floodplain formation began by the mid-1930s, approximately the same time that the flow regime decreased in total annual and peak annual flow. Tamarisk, a nonnative shrub, began to establish in the 1930s as well. Narrowing from the 1940s to the mid-1980s was insignificant, because floodplain formation was approximately matched by bank erosion. Air photo analysis demonstrates that the most significant episode of narrowing was underway by the late 1980s, and analysis of the trench shows that floodplain formation had begun in the mid-1980s during a multi-year period of low peak annual flow. Air photo analysis shows that mean channel width decreased by ∼7% between 1993 and 2009. A new phase of narrowing may have begun in 2003, based on evidence in the trench. Comparison of field surveys made in 1998 and 2015 in an 8.5 km reach near Fort Bottom suggests that narrowing continues and demonstrates that new floodplain formation has been a very small proportion of the total annual fine sediment flux of the Green River. Vertical accretion of new floodplains near Fort Bottom averaged 2.4 m between 1998 and 2015 but only accounted for ∼1.5% of the estimated fine sediment flux during that period. Flood control by Flaming Gorge Dam after 1962 significantly influenced flow regime, reducing the magnitude of the annual snowmelt flood and increasing the magnitude of base flows. Though narrowing was initiated by changes in flow regime, native and nonnative riparian vegetation promoted floodplain formation and channel narrowing especially through establishment on channel bars and incipient floodplains during years of small annual floods.

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Analyses of suspended sediment transport provide valuable insight into the role that sediment supply plays in causing geomorphic change. The sediment supply within a river system evolves depending on the discharge, flood frequency and duration, changes in sediment input, and ecohydraulic conditions that modify sediment transport processes. Changes in supply can be evaluated through analyses of coupled changes in suspended sediment concentration and grain size. The concentration of sand in transport in the Green and Colorado Rivers is most strongly controlled by discharge and the bed sand grain size distribution. Since the 1950s, sand loads have decreased in response to declines in peak discharge in the Green River and coarsening of the bed sand in the Colorado River. However, changes in the bed sand grain size distribution are associated with large changes in suspended sand concentration in both rivers; concentration varies by a factor of~3 in the Green River and a factor of~8 in the Colorado River, depending on the bed sand grain size distribution. Analyses of hysteresis in suspended sediment measurements show that sediment depletion during annual floods is most strongly controlled by flood duration, with peak discharge being nearly equally important in the Green River. Despite channel narrowing in both rivers, periods of bed sand coarsening and sediment depletion during annual floods indicate that these rivers are not necessarily in sediment surplus. Channel narrowing appears to be strongly controlled by short-term declines in flood magnitude and the ecohydraulic effects of vegetation and may not be indicative of the long-term sediment budget. Plain Language Summary River channels change size and shape in response to changes in the amount of sediment transported downstream. Changes in streamflow and/or the upstream sediment supply are the cause(s) of such changes in sediment transport. The channels of the Green and Colorado Rivers near Canyonlands National Park, Utah, have both narrowed over the last century. We use measurements of suspended sediment transport to investigate how changes in the sediment supply influence sediment transport and channel change. In most cases, the transport of suspended sand is primarily controlled by the discharge of water and secondarily controlled by the bed sediment grain size distribution. Depletion of the upstream sand supply leads to bed sand coarsening and erosion, whereas enrichment of the upstream sand supply leads to bed sand fining and deposition. The sand supply is progressively depleted during annual snowmelt floods on the Green and Colorado Rivers, with greater depletion occurring during longer floods. Larger floods also cause greater depletion of the upstream sand supply in the Green River but are of less importance in the Colorado River. The size and shape of the present-day river channels may therefore be maintained, and channel narrowing may be limited, if longer-duration floods occur in the future.

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This paper uses a hedonic price model to estimate the impact of water level on the value of real estate on Lake Koshkonong in Wisconsin. Hedonic techniques are employed to show that a reduction in the lake’s water level has a significant effect on shoreline property values. The body of existing research demonstrates that changes in both the subjective and objective indicators of value are important for estimating the implicit value of water quality in hedonic analysis. This paper provides new evidence on the economic harm to lake communities created by the reduction of water levels.