Channel narrowing by inset floodplain formation of the lower Green River in the Canyonlands region, Utah

Abstract

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.

Does Channel Narrowing by Floodplain Growth Necessarily Indicate Sediment Surplus? Lessons From Sediment Transport Analyses in the Green and Colorado Rivers, Canyonlands, Utah

“…In the Green River, Allred and Schmidt (1999) showed that channel narrowing by ~17 m occurred at the upper end of our study area between 1962 and 1985 by vertical aggradation of ~1.1 m of sediment on top of bars within the active channel; aggradation occurred in 12 of the years in this period. Walker et al (2020) showed that the Green River within and below the study area narrowed by ~12 m between the mid‐1980s and 2014 by the vertical aggradation of ~1.5 m of sediment also on top of active channel bars. These numbers indicate that ~1.5 and ~2 million metric tons of sand accumulated along the ~110‐km river segment in our study area (GR‐GR to GR‐MB) between 1962 and 1985 and the mid‐1980s and 2014, respectively.…”

“…These quantities are equivalent to ~3% and ~4% of the average annual sand load over these respectively time periods or ~1% of the sand load prior to 1959. Thus, channel margin and floodplain deposition of only a small amount of the annual load is required to achieve the observed channel narrowing (Walker et al, 2020). This is consistent with the findings of Grams and Schmidt (2005) that floodplain accretion in Dinosaur National Monument was ~1% of the annual sand load.…”

“…Channel narrowing in the Green River began as early as the 1930s when peak Q declined, greatly accelerated in the late 1980s, and continued through at least 2014 (Walker et al, 2020). In the study area, most of the channel narrowing in the Green River has occurred since the late 1980s (Walker et al, 2020). Narrowing occurred through the growth of in‐channel bars, stabilization of these bars by vegetation during years with small annual floods, and the subsequent conversion of these bars to floodplains.…”

Quantifying isolation-by-resistance and connectivity in dendritic ecological networks

“…6; Table 2). Several of these are functionally-related, such as: Agricultural/ urban areas as major drivers of water usage (USE); flow regulation/ flood dampening (DOR) as controlling sediment supply (SED) as well as channel width and morphology (Grams et al 2020, Walker et al 2020).…”

The importance of threshold in alluvial river channel geometry and dynamics

“…channel adjustment may result principally from changes in the frequency and intensity of sediment transport 192 and therefore be amenable to a mechanics based solution, other factors may also contribute. For example, interannual vegetation growth -along channel banks, on bars, and within the upstream catchment -could contribute significantly both in changing the entrainment threshold, and also by potentially introducing lags and hysteresis in channel response 193,194 .…”