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How Big of an Effect Do Small Dams Have? Using Geomorphological Footprints to Quantify Spatial Impact of Low-Head Dams and Identify Patterns of Across-Dam Variation.

Fencl JS, Mather ME, Costigan KH, Daniels MD - PLoS ONE (2015)

Bottom Line: Dams are significant disruptions to streams.Both characteristics of individual dams and the context of neighboring dams affected low-head dam impacts within the river network.For these reasons, low-head dams require a different, more integrative, approach for research and management than the individualistic approach that has been applied to larger dams.

View Article: PubMed Central - PubMed

Affiliation: Kansas Cooperative Fish and Wildlife Research Unit, Division of Biology, Kansas State University, Manhattan, Kansas, United States of America.

ABSTRACT
Longitudinal connectivity is a fundamental characteristic of rivers that can be disrupted by natural and anthropogenic processes. Dams are significant disruptions to streams. Over 2,000,000 low-head dams (<7.6 m high) fragment United States rivers. Despite potential adverse impacts of these ubiquitous disturbances, the spatial impacts of low-head dams on geomorphology and ecology are largely untested. Progress for research and conservation is impaired by not knowing the magnitude of low-head dam impacts. Based on the geomorphic literature, we refined a methodology that allowed us to quantify the spatial extent of low-head dam impacts (herein dam footprint), assessed variation in dam footprints across low-head dams within a river network, and identified select aspects of the context of this variation. Wetted width, depth, and substrate size distributions upstream and downstream of six low-head dams within the Upper Neosho River, Kansas, United States of America were measured. Total dam footprints averaged 7.9 km (3.0-15.3 km) or 287 wetted widths (136-437 wetted widths). Estimates included both upstream (mean: 6.7 km or 243 wetted widths) and downstream footprints (mean: 1.2 km or 44 wetted widths). Altogether the six low-head dams impacted 47.3 km (about 17%) of the mainstem in the river network. Despite differences in age, size, location, and primary function, the sizes of geomorphic footprints of individual low-head dams in the Upper Neosho river network were relatively similar. The number of upstream dams and distance to upstream dams, but not dam height, affected the spatial extent of dam footprints. In summary, ubiquitous low-head dams individually and cumulatively altered lotic ecosystems. Both characteristics of individual dams and the context of neighboring dams affected low-head dam impacts within the river network. For these reasons, low-head dams require a different, more integrative, approach for research and management than the individualistic approach that has been applied to larger dams.

No MeSH data available.


Related in: MedlinePlus

Predictions about Dam Impact.Predictions of geomorphic effects caused by low-head dams on (A) wetted width and depth, (B) channel widening, and (C) substrate size from the Web of Science literature on low-head dams. On all prediction plots, the X axis is the distance from the dam, the black trapezoid represents dam position, the area left of the dashed line represents habitat upstream of the dam and the area right of the dashed line represents habitat downstream of the dam. The impoundment is represented by grey shading.
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pone.0141210.g001: Predictions about Dam Impact.Predictions of geomorphic effects caused by low-head dams on (A) wetted width and depth, (B) channel widening, and (C) substrate size from the Web of Science literature on low-head dams. On all prediction plots, the X axis is the distance from the dam, the black trapezoid represents dam position, the area left of the dashed line represents habitat upstream of the dam and the area right of the dashed line represents habitat downstream of the dam. The impoundment is represented by grey shading.

Mentions: Specifically, the geomorphic literature predicts three major changes around low-head dams related to dam-induced alterations to flow and sediment regimes (Fig 1). First, the backwater effect of dams creates ponding in the upstream reservoir, producing wetted stream widths and depths greater than downstream of the dam, with the spatial extent of these impacts entirely dependent upon local system channel geometry, channel slope, and height of the dam (Fig 1A) [13, 14, 15]. The combination of backwater ponding effects and partial sediment excavation during high flows in the impoundment are thought to maintain these greater depths and prevent complete sediment infilling of the backwater zone [16]. Second, the combination of some sediment trapping in the impoundment during low flows and the high energy acceleration as flow drops over low-head dams produces scour of the bed and banks, in some cases producing a deep plunge pool and mid-channel bar comprised of coarse scoured material immediately downstream of the low-head dam (Fig 1B). This scour-deposition pattern is caused by transport effective high flows that move sediments through the impoundment and excess energy in the flow that dissipates quickly once the water transits the hydraulic jump at the dam face [16]. If a mid-channel bar does form a short distance downstream from a dam, this may further contribute to channel widening by deflecting flow toward the banks (Fig 1B) [15, 16, 17]. Third, the enhanced flow energy and partial clear water effect immediately downstream of the dam during low and moderate flows induces mobilization of fine fractions of the substrate, producing a coarsening of the substrate below low-head dams, leaving only coarse material (cobble, boulder, bedrock) behind (Fig 1C) [15, 18]. Further downstream of the dam, substrate particle size distributions are assumed to return to the downstream fining pattern in undammed systems (e.g., [19, 20, 21]), but the spatial extent of this adjustment is not well understood (Fig 1C).


How Big of an Effect Do Small Dams Have? Using Geomorphological Footprints to Quantify Spatial Impact of Low-Head Dams and Identify Patterns of Across-Dam Variation.

Fencl JS, Mather ME, Costigan KH, Daniels MD - PLoS ONE (2015)

Predictions about Dam Impact.Predictions of geomorphic effects caused by low-head dams on (A) wetted width and depth, (B) channel widening, and (C) substrate size from the Web of Science literature on low-head dams. On all prediction plots, the X axis is the distance from the dam, the black trapezoid represents dam position, the area left of the dashed line represents habitat upstream of the dam and the area right of the dashed line represents habitat downstream of the dam. The impoundment is represented by grey shading.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4634923&req=5

pone.0141210.g001: Predictions about Dam Impact.Predictions of geomorphic effects caused by low-head dams on (A) wetted width and depth, (B) channel widening, and (C) substrate size from the Web of Science literature on low-head dams. On all prediction plots, the X axis is the distance from the dam, the black trapezoid represents dam position, the area left of the dashed line represents habitat upstream of the dam and the area right of the dashed line represents habitat downstream of the dam. The impoundment is represented by grey shading.
Mentions: Specifically, the geomorphic literature predicts three major changes around low-head dams related to dam-induced alterations to flow and sediment regimes (Fig 1). First, the backwater effect of dams creates ponding in the upstream reservoir, producing wetted stream widths and depths greater than downstream of the dam, with the spatial extent of these impacts entirely dependent upon local system channel geometry, channel slope, and height of the dam (Fig 1A) [13, 14, 15]. The combination of backwater ponding effects and partial sediment excavation during high flows in the impoundment are thought to maintain these greater depths and prevent complete sediment infilling of the backwater zone [16]. Second, the combination of some sediment trapping in the impoundment during low flows and the high energy acceleration as flow drops over low-head dams produces scour of the bed and banks, in some cases producing a deep plunge pool and mid-channel bar comprised of coarse scoured material immediately downstream of the low-head dam (Fig 1B). This scour-deposition pattern is caused by transport effective high flows that move sediments through the impoundment and excess energy in the flow that dissipates quickly once the water transits the hydraulic jump at the dam face [16]. If a mid-channel bar does form a short distance downstream from a dam, this may further contribute to channel widening by deflecting flow toward the banks (Fig 1B) [15, 16, 17]. Third, the enhanced flow energy and partial clear water effect immediately downstream of the dam during low and moderate flows induces mobilization of fine fractions of the substrate, producing a coarsening of the substrate below low-head dams, leaving only coarse material (cobble, boulder, bedrock) behind (Fig 1C) [15, 18]. Further downstream of the dam, substrate particle size distributions are assumed to return to the downstream fining pattern in undammed systems (e.g., [19, 20, 21]), but the spatial extent of this adjustment is not well understood (Fig 1C).

Bottom Line: Dams are significant disruptions to streams.Both characteristics of individual dams and the context of neighboring dams affected low-head dam impacts within the river network.For these reasons, low-head dams require a different, more integrative, approach for research and management than the individualistic approach that has been applied to larger dams.

View Article: PubMed Central - PubMed

Affiliation: Kansas Cooperative Fish and Wildlife Research Unit, Division of Biology, Kansas State University, Manhattan, Kansas, United States of America.

ABSTRACT
Longitudinal connectivity is a fundamental characteristic of rivers that can be disrupted by natural and anthropogenic processes. Dams are significant disruptions to streams. Over 2,000,000 low-head dams (<7.6 m high) fragment United States rivers. Despite potential adverse impacts of these ubiquitous disturbances, the spatial impacts of low-head dams on geomorphology and ecology are largely untested. Progress for research and conservation is impaired by not knowing the magnitude of low-head dam impacts. Based on the geomorphic literature, we refined a methodology that allowed us to quantify the spatial extent of low-head dam impacts (herein dam footprint), assessed variation in dam footprints across low-head dams within a river network, and identified select aspects of the context of this variation. Wetted width, depth, and substrate size distributions upstream and downstream of six low-head dams within the Upper Neosho River, Kansas, United States of America were measured. Total dam footprints averaged 7.9 km (3.0-15.3 km) or 287 wetted widths (136-437 wetted widths). Estimates included both upstream (mean: 6.7 km or 243 wetted widths) and downstream footprints (mean: 1.2 km or 44 wetted widths). Altogether the six low-head dams impacted 47.3 km (about 17%) of the mainstem in the river network. Despite differences in age, size, location, and primary function, the sizes of geomorphic footprints of individual low-head dams in the Upper Neosho river network were relatively similar. The number of upstream dams and distance to upstream dams, but not dam height, affected the spatial extent of dam footprints. In summary, ubiquitous low-head dams individually and cumulatively altered lotic ecosystems. Both characteristics of individual dams and the context of neighboring dams affected low-head dam impacts within the river network. For these reasons, low-head dams require a different, more integrative, approach for research and management than the individualistic approach that has been applied to larger dams.

No MeSH data available.


Related in: MedlinePlus