Reservoirs and streamflow

Reservoirs are artificial lakes produced by dams that impede streamflow. They function like lakes in almost all respects, but reservoirs are located by human design and are usually operated to achieve multiple goals. Reservoirs mimic natural lakes regarding their hydroclimatic influences on water storage and evaporation, and their water balance is expressed by Equation 6.14. However, flow regulation is more evident with reservoirs as the storage volume is managed to accommodate flood control, navigation, irrigation, hydroelectric production, recreation, water supply, water quality, fish and wildlife requirements, and other conservation purposes. The regulated streamflow masks much of the natural discharge variability and alters the natural discharge quantity to such an extent that observed streamflow may bear little resemblance to the natural flow.

Reservoirs are typically operated as storage or run-of-river impoundments. Storage reservoirs have sufficient capacity to offset seasonal streamflow fluctuations and provide relatively constant flow throughout the year. Run-of-river impoundments use low dams to create a pool that stores a daily or weekly

6.14 Reservoirs and streamflow 205

Fig. 6.14. Daily mean streamflow for the North Santiam River at Niagra, Oregon (45° N), downstream from Detroit Reservoir. The bold line is outflow due to reservoir operations, and the gray line is the gauged reservoir inflow. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

water volume used primarily for hydropower production or navigation. These dams cause few alterations to natural streamflow. Storage reservoirs have the greatest impact on the hydrologic cycle and streamflow, and large storage reservoirs alter streamflow the most.

The North Santiam River in western Oregon displays the impact of a storage reservoir on natural streamflow (Fig. 6.14). Detroit Dam (45° N) creates a multipurpose reservoir with a storage of 0.396 km3. The 1132 km2 watershed above the dam produces a mean daily streamflow of 59m3s_1. Reservoir operations reduce high flows as seen by the 205 m3 s_1 difference between inflow and outflow peaks in late 1999 and the 100 m3 s_1 difference in April 2002. Maintaining low outflows greater than inflows is consistently demonstrated during the exceedingly dry period from July 2000 to November 2001. Precipitation from October to February was as little as 25% of average within the watershed, and the 12 months from October 2000 through September 2001 were the second driest period on record for many Oregon stations.

Complex influences on streamflow are evident when reservoir storage is operated to support multiple purposes and numerous water diversions are factors. Streamflow in the Sacramento River at Freeport, California, (38° N) is altered by multiple human influences on streamflow related to the operation of 43 major reservoirs, a major water diversion into the basin, and several diversions out of the basin (Shelton, 1995). The Sacramento River watershed at Freeport drains nearly 68 000 km2, and records of both gauged and calculated streamflow are available for this site for the years 1949 to 1994. The gauged record at Freeport is combined with the gauged record for the Yolo Bypass to represent regulated

Fig. 6.15. Mean monthly streamflow for the Sacramento River at Freeport, California (38° N). Shaded columns are the observed flow regulated by reservoir operations and irrigation and flood diversions. Stippled columns are the calculated natural flow determined by adjusting observed flow for the influences of reservoir operations, evaporation, and irrigation and flood diversions. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

streamflow. The Yolo Bypass is a flood control feature that protects the city of Sacramento. Water is diverted from the Sacramento River above the city and flows through a levee-bounded area that rejoins the river below Freeport. This facility is dry for many months each year, and it is used only when flows exceed a potentially damaging stage which does not occur in all years. Calculated stream-flow is employed as an estimate of natural streamflow by adjusting measured streamflow for changes in reservoir storage, known or estimated channel losses, evaporation, and water diversions into or out of the watershed. These adjustments attempt to account for human intervention in the runoff process.

The average annual calculated streamflow for the Sacramento River is 27136 km3, and 79% of this flow occurs in December through May (Fig. 6.15). Winter precipitation and spring snowmelt in the alpine regions of the watershed contribute to the March calculated streamflow being 7.7-times greater than September streamflow. The 46 years of regulated streamflow indicate an average annual value of 23417 km3. The difference between calculated and regulated streamflow is due largely to diversions for irrigation and urban use which increase evapotranspiration. This influence is evident during July through October when regulated streamflow exceeds calculated streamflow.

Annual variations in the relationship between calculated and regulated streamflow (Fig. 6.16) reflect the complex hydroclimate of the Sacramento River watershed and the changing character of human influences during the 46 years. Both data series show 1983 is the wettest year and 1977 is the driest year. The difference between calculated and regulated streamflow is smallest in 1977 and is

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Month

70 000 j

-H 50 0005

-H 50 0005

1949 1955 1961 1967 1973 1979 1985 1991 Year

Fig. 6.16. Annual streamflow for the Sacramento River at Freeport, California (38° N). Solid columns are the observed flow regulated by reservoir operations and irrigation and flood diversions. Gray columns are the calculated natural flow determined by adjusting observed flow for the influences of reservoir operations, evaporation, and irrigation and flood diversions. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

1949 1955 1961 1967 1973 1979 1985 1991 Year

Fig. 6.16. Annual streamflow for the Sacramento River at Freeport, California (38° N). Solid columns are the observed flow regulated by reservoir operations and irrigation and flood diversions. Gray columns are the calculated natural flow determined by adjusting observed flow for the influences of reservoir operations, evaporation, and irrigation and flood diversions. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

greatest in 1978. Calculated streamflow is larger than regulated streamflow in all years except 1976 and 1994. These are the second and third driest years in the 46-year record, and reservoirs were drawn down with the expectation that winter precipitation would replenish storage. Meager winter precipitation in 1977 did not replenish reservoir storage, and 1977 regulated streamflow was reduced to 29% of the average annual regulated streamflow. Abundant precipitation in 1978 produced a striking contrast as regulated streamflow was 44% of the average and calculated streamflow was 136% of the average and the 11th highest annual calculated streamflow in the 46 years. The streamflow disparity in 1978 reflects efforts to refill surface reservoir storage and to increase diversions to restore depleted soil moisture for agriculture (Shelton, 1995).

Other influences of storage reservoirs include disrupting natural flood cycles, disconnecting rivers from wetlands and floodplains, disrupting fish migrations, altering riverine and terrestrial habitats, and altering downstream sediment deposition. Sediment accumulation in storage reservoirs reduces their water volume, and the reduced sediment delivery by rivers to the oceans alters coastal processes (Syvitski et al., 2005).

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