The rate and pattern of bed incision and bank adjustment on the Colorado River in Glen Canyon downstream from Glen Canyon Dam, 1956-2000

Abstract

Closure of Glen Canyon Dam in 1963transformed the Colorado River by reducing the magnitude and duration of spring fl oods, increasing the magnitude of base fl ows, and trapping fi ne sediment delivered from the upper watershed. These changes caused the channel downstream in Glen Canyon to incise, armor, and narrow. This study synthesizes over 45 yr of channel-change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment defi cit and human decisions about dam operations. Although bed lowering in lower Glen Canyon began when the fi rst cofferdam was installed in 1959, most incision occurred in 1965 in conjunction with 14 pulsed high fl ows that scoured an average of 2.6 m of sediment from the center of the channel. The average grain size of bed material has increased from 0.25 mm in 1956 to over 20 mm in 1999. The magnitude of incision at riffl es decreases with distance downstream from the dam, while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream. Analysis of bed-material mobility indicates that the increase in bed-material grain size and reduction in reach-average gradient are consistent with the transformation of an adjustable-bed alluvial river to a channel with a stable bed that is rarely mobilized.Decreased magnitude of peak discharges in the post-dam regime coupled with channel incision and the associated downward shifts of stage-discharge relations have caused sandbar and terrace erosion and the transformation of previously active sandbars and gravel bars to abandoned deposits that are no longer inundated. Erosion has been concentrated in a few pre-dam terraces that eroded rapidly for brief periods and have since stabilized. The abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shift in the stage-discharge relations. In the downstream part of the study area where riffl es controlling channel elevation have not incised, channel narrowing has resulted from decreased magnitude of peak discharges and minor post-dam deposition. These physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non-native species.Models for the channel response downstream from large dams typically consider factors such as the degree of sediment defi cit, the pre-dam surface and subsurface grain size, and the magnitude of post-dam average fl ows. These results suggest that it is also necessary to consider (1) the possibility of variable responses among different channel elements and (2) the potential importance of exceptional fl ows resulting from management decisions.

Temporal and spatial scales of geomorphic adjustments to reduced competency following flow regulation in bedload-dominated systems

“…Results from studies of large dams, particularly those in the Colorado River basin of the western U.S. (Andrews, 1990;Schmidt et al, 1995;Pitlick and Van Steeter, 1998;Allred and Schmidt, 1999;Topping et al, 2000aTopping et al, ,b, 2007Grams et al, 2007) demonstrate that the geomorphic impacts of flow regulation are often strongly controlled by the balance between sediment supply and the sediment transport capacity of the river. Sediment trapping by a dam, for example, results in greater sediment transport capacity than supply immediately downstream of the dam; this imbalance, in turn, leads to channel incision (Williams and Wolman, 1984) and a narrowing of the bankfull channel width (Andrews, 1986).…”

“…The abandonment of pre-dam floodplains results in their subsequent re-vegetation and the development of new, lower terraces that further reduce the channel width (Petts, 1984;Grams and Schmidt, 2002;Magilligan and Nislow, 2005). Evacuation of fine sediment associated with incision may also lead to sandbar erosion (Grams et al, 2007;Wright et al, 2008), decreased embeddedness (Salant et al, 2006), and decreased ecological diversity and health of native benthic species (Svendsen et al, 2009). Farther downstream, the impact of the dam is modified as the supply of newly derived sediment from downstream tributaries, in addition to sediment removed via evacuation upstream, ultimately exceeds the reduced sediment transport capacity of the regulated mainstem, leading to channel aggradation (Graf, 1980;Andrews, 1986;Everitt, 1993;Grant et al, 2003;Salant et al, 2006;Svendsen et al, 2009).…”

Invasion and production of New Zealand mud snails in the Colorado River, Glen Canyon

“…1) with minimal diel fluctuation (0.5-1°C). River width and thalweg depth average *120 and 7 m at a discharge of 325 m 3 /s, respectively (Grams et al 2007;P.E. Grams, unpublished data, USGS, Flagstaff, AZ), but a large amount of variability exists due to fluctuating discharge.…”

Sediment transport constraints on river response to regulation

“…Previous research has well documented the broad suite of channel changes following impoundment, including post-dam channel narrowing (Friedman et al, 1998;Allred and Schmidt, 1999;Surian, 1999;Grams et al, 2007), reduced channel capacity (Chin et al, 2002), bed incision (Andrews, 1986;Surian and Rinaldi, 2003;Vericat and Batalla, 2005;Grams et al, 2007;Surian and Cisotto, 2007), sinuosity adjustments (Magilligan et al, 2008), and bar growth, especially at tributary junctions (Petts and Thoms, 1987;Curtis et al, 2010). Despite the voluminous case studies documenting the range of channel adjustments to flow regulation, predicting the specific direction and magnitude of geomorphic adjustments remains unresolved (Maddock, 1970).…”

“…The disruption of sediment flux associated with flow regulation (or dam removal) is known to alter the morphology of rivers (Williams and Wolman, 1984;Andrews, 1986;Church, 1995;Brandt, 2000;Salant et al, 2006;Grams et al, 2007) and the benthic ecology (Petts and Greenwood, 1985;Wootton et al, 1996;Rice et al, 2001Rice et al, , 2006Svendsen et al, 2009), but in as yet poorly understood or predictable ways (Maddock, 1970). Previous research has well documented the broad suite of channel changes following impoundment, including post-dam channel narrowing (Friedman et al, 1998;Allred and Schmidt, 1999;Surian, 1999;Grams et al, 2007), reduced channel capacity (Chin et al, 2002), bed incision (Andrews, 1986;Surian and Rinaldi, 2003;Vericat and Batalla, 2005;Grams et al, 2007;Surian and Cisotto, 2007), sinuosity adjustments (Magilligan et al, 2008), and bar growth, especially at tributary junctions (Petts and Thoms, 1987;Curtis et al, 2010).…”