Analysis of bank erosion on the Merced River, Yosemite Valley, Yosemite National Park, California, USA

Abstract

Channel changes from 1919 to 1989 were documented in two study reaches of the Merced River in Yosemite National Park through a review of historical photographs and documents and a comparison of survey data. Bank erosion was prevalent and channel width increased an average of 27% in the upstream reach, where human use was concentrated. Here, trampling of the banks and riparian vegetation was common, and banks eroded on straight stretches as frequently as on meander bends. Six bridges in the upper reach constrict the channel by an average of 38% of the original width, causing severe erosion. In the downstream control reach, where human use was minimal, channel widths both decreased and increased, with a mean increase of only 4% since 1919. Bank erosion in the control reach occurred primarily on meander bends, The control reach also had denser stands of riparian vegetation and a higher frequency of large woody debris in channels. There is only one bridge in the lower reach, located at the downstream end. Since 1919, bank erosion in the impacted upstream reach contributed a significant amount of sediment (74,800 tonnes, equivalent to 2.0 t/km2/yr) to the river. An analysis of 75 years of precipitation and hydrologic records showed no trends responsible for bank erosion in the upper reach. Sediment input to the upper reach has not changed significantly during the study period. Floodplain soils are sandy, with low cohesion and are easily detached by lateral erosion. The degree of channel widening was positively correlated with the percentage of bare ground on the streambanks and low bank stability ratings. Low bank stability ratings were, in turn, strongly associated with high human use areas. Channel widening and bank erosion in the upper reach were due primarily to destruction of riparian vegetation by human trampling and the effect of bridge constrictions on high flow, and secondarily to poorly installed channel revetments. Several specific recommendations for river restoration were provided to park management.

Opportunities and challenges for restoration of the Merced River through Yosemite Valley, Yosemite National Park, USA

“…We characterized the size and position of the past and (projected) future Merced River channel through multiple means. Channel widths between 1919 (the first USGS topographic map) and 1977 were compiled by Milestone (1978), with an additional decade of data added by Madej et al (1991). Channel positions through 2011 were compiled from old maps and airphotos by Cardno (2012) into GIS; both widths and positions were subsequently updated by additional airphotos through 2018 for the present study.…”

“…Madej, Weaver, and Hagans (1991) compared measurements of channel width made in 1986 and 1989 with USGS topographic maps from 1919, documenting both pervasive channel widening, particularly in areas with much bare ground associated with high human use, and channel constrictions associated with the bridges. More recently, Cardno (2012) inventoried channel and riparian conditions throughout the Valley.…”

Plants as river system engineers

“…() stressed the importance of sediment trapping by colonizing vegetation in inducing channel narrowing following major erosive flood events along Plum Creek, Colorado; Kondolf and Curry (, ) and Madej et al . () described decreased bank stability and channel widening (including transitions from meandering to braiding planforms) as a result of a decrease in riparian vegetation biomass along the Carmel and Merced Rivers, California, USA; and Graf (, ) and Rowntree () identified channel narrowing associated with increased sediment trapping and reinforcement by alien riparian woody plant species invading river corridors in southwest USA and South Africa, respectively. Riparian vegetation was also noted as an influence on the erosion resistance and stability of levées (e.g.…”

Bank erosion along the dam-regulated lower Roanoke River, North Carolina

“…Bank-erosion rates on the Roanoke River (0.52 m/yr maximum) are similar to other published erosion rates (relatively rare in the literature) where human activities have affected natural channel processes. Madej et al (1994) documented erosion rates of ~0.51 m/yr along a reach of the Merced River, California, that was severely impacted by concentrated human recreational development including bank armoring. Maximum channel widening rates of 1.1 m/yr were documented below a dam on the Green River, Colorado (Merritt and Cooper, 2000).…”