Chapter 3 : Sedimentation

3.1 Introduction

Figure 3.1. Matilija Dam, on the Ventura River Basin, California, was constructed in 1947 and is nearly completely filled with sediment. When constructed, the dam was 190 ft high and impounded a volume of 7,000 ac-ft; by 2015 it was reduced to 400 ac-ft. Photo credit: P. Jenkin.

Sedimentation is a natural process in all water bodies. Sedimentation is relatively higher in reservoirs than in other water bodies because reservoirs impound a large volume relative to the area of their watershed. Sediment accumulation is accelerated by inadequate land-use practices that liberate soils, by the conversion of land into urban and suburban development that hastens runoff, or both. The rivers and streams deposit their sediment loads in the calmer waters of reservoirs, where sediment  accumulation  can have negative effects. Infilling with sediment can decrease water storage capacity and reduce the benefits of storing water in reservoirs. Shallower waters also may decrease the recreational value of a reservoir and the loss of access to parts of the upper reaches and embayments. Sedimentation also can result in the loss of habitat for fish, and sediment can carry pollutants    including   nutrients, which may act as catalysts for eutrophication.

Reservoir sedimentation can change physical, chemical, and biological components of the ecosystem, which results in the degradation of beneficial uses such as drinking water supplies, navigation, electricity production, flood control, and recreation (Figure 3.1). Eventually the reservoir may have to be abandoned. In the USA more than 3,000 such dams have been retired (Marsh 2005). The effects of deposited sediment delivered from watersheds can have severe economic costs for downstream residents and may decrease property values for lakefront properties and those properties near the reservoir.

Sedimentation is a major issue in many reservoirs in the USA. A survey of reservoir managers identified that approximately 28% of reservoirs >250 ac in the USA were of moderate-to-high or high concern relative to sedimentation (Krogman and Mi- randa 2016). These percentages vary regionally; for example, sedimentation afflicts as many as 51% of reservoirs in regions along the plains in the central USA. This same survey identified various correlations between sedimentation and both watershed and in-lake characteristics, particularly turbidity and loss of shallow reservoir habitats (Table 3.1).

Table 3.1. Spearman correlations (rs) between sedimentation and various watershed and in-lake characteristics. All  correlations are statistically significant (P < 0.01; N = 1,271 reservoirs).
Variable Description rs
Harmful levels of agriculture Watershed around the reservoir has adverse row-crop agriculture practices 0.54
Harmful levels of livestock Watershed around the reservoir has adverse grazing practices and/or  feedlot production 0.45
Disturbances in upstream watersheds Disturbances in watersheds upstream of the reservoir (not around) impairs habitat 0.55
Lack of connectivity due to sedimentation Sedimentation has decreased connectivity to tributaries during low flow, acting as a barrier to fish movement 0.49
Shoreline erosion Removal of soil and terrestrial vegetation from the land–water interface resulting from weathering of banks or adjacent land slopes by water, ice, wind, or other 0.63
Shoreline homogenization A reduction of the shoreline's original habitat diversity by erosion or other processes 0.52
Homogenization of littoral substrates A reduction of the substrate's original diversity by erosion and sedimentation 0.63
Excessively shallow Reservoir is excessively shallow with no or few deep water refuges 0.54
Excessive mudflats Seasonally flooded and exposed expansive soft sediment present; terrestrial vegetation seldom grows unless mudflats are exposed for many months 0.55
Excessively shallow littoral zone Littoral zone is mostly shallow and heavily influenced by temperature, wind, and other atmospheric changes 0.56
Excessive nutrients Excessive nutrients, primarily nitrogen or phosphorous, that may increase primary production and lead to excessive plant growth and decay and, lack of oxygen 0.55
Excessive suspended sedi- ment or inorganic turbidity Particulate inorganic matter and fine sediment in the water column that may inhibit primary production or foraging by fish and other aquatic organisms 0.72
Excessive organic turbidity Particulate organic matter, other than algal blooms, suspended in the water column 0.56
Loss of cove habitat due to sedimentation Sedimentation has changed cove habitat, including reduced surface area, fragmentation, and establishment of terrestrial vegetation in newly deposited land 0.73

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3.2 Sources of Sediment

Reservoir sedimentation begins with soil erosion caused by rain and wind and with runoff that transports sediment particles into streams (Novotny and Olem 1994). Depending on composition, various types of land cover produce different runoff characteristics. Determining sediment sources is essential for designing cost-effective sediment management strategies that will achieve meaningful reductions in sediment loads and yields (Walling 2005). Overall, the sediment entering reservoirs originates from erosion of four general sources: (1) soil from overland flow, including farmed areas in the watershed; (2) streambank and channel erosion, including channel migra- tion, bank widening, and avulsion; (3) remobilization of stored sediment through channel processes acting on floodplains or other storage sites; and (4) erosion of shorelines and shallow-water areas by wave action.

Streamflow slows as it enters a reservoir, and suspended particles begin to settle out. Eventually, most sediment will settle to the bottom of the reservoir, but heavier sediment particles are deposited first. Sedimentation does not occur uniformly; it is affected by many factors including the flow and volume of water produced by the incoming stream and the size and weight of sediment particles. The coarser portion of the inflowing sediment load is deposited where the main tributary and minor tributaries to embayments enter the reservoir. There, the tributaries form delta deposits that deplete reservoir storage, cause channel aggradation extending miles up- stream from the reservoir, and fill in shallow coves and embayments that often represent some of the most diverse fish habitats in the reservoir (Williams 1991). Channel aggradation can change flood patterns and floodplain configuration upstream. If delta areas become heavily vegetated, the upstream flood levels can be elevated further be- cause of increased hydraulic roughness, and the vegetation can trap sediment, thereby promoting additional aggradation (James and Barko 1990). In arid zones the transpiration from large areas of vegetation in delta areas can increase water losses from the reservoir significantly. For example, evaporative losses from the delta of Elephant Butte Reservoir on the Rio Grande in New Mexico were estimated at 140,000 ac-ft/year before they were reduced by 66,000 ac-ft/year by the construction of a low-flow conveyance channel through the delta in 1951 (BOR 2007).

The nature of the sediment accumulated in a reservoir depends on geology, topography, soil, and climatic conditions. Where parent materials in the watershed are shales or limestone, sand content of sediment is low. Where parent material is mainly sandstone, sand content may be high. Some igneous and metamorphic rocks produce fine sediment under some climatic conditions and coarse material under others. For example, sediment derived from Piedmont areas in the southeastern USA contains proportionally large amounts of clay and colloidal material. Sediment derived from loess-type soils in the Midwest has high silt content. The Cross Timbers area of Texas, with sandy soils and poorly consolidated sandstone substrata, provides sediment with high sand content. Environmental origin has a definite bearing on watershed sediment yield, transport, and deposition in a reservoir, and the nature of the sediment has a direct bearing on the percentage of total load deposited in the reservoir and on the ultimate volume of deposited material.

A river can carry sediment into a reservoir in two distinct modes: bedload and suspended particles. In bedload transport, the sediment particles move by rolling or sliding or through jumps the length of a few grain sizes (known as saltation), and they are thus in frequent contact with the channel bed. In suspended load transport, the weight of the particles is supported by turbulent forces in the water, and they can travel considerable distances without coming into contact with the bed. The total load is the sum of suspended load and bedload. Whether an individual particle is trans- ported as suspended load or as bedload depends on particle size, weight, and shape and on hydraulic conditions. Whereas the bedload is often deposited in the upper end of reservoirs or the upper end of embayments, depending on current suspended loads can move farther into the reservoir. This fraction does not easily sink in the water column, and slight turbulent forces keep it in suspension for long periods. In rare in- stances, light dispersive colloidal clays delivered in suspended loads never settle out and remain in suspension aided by minimal wind-generated wave energy. In reservoirs, besides river inputs, fine suspended material originates from shoreline erosion and organic and inorganic material generated within the reservoir by biological activity. In eutrophic waters the latter source can be quite significant. Fine material can be resuspended repeatedly by currents and wave action until it eventually is deposited in an area where water movements are insufficient to resuspend or remobilize it.
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3.3 Stages in Reservoir Sedimentation

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