CONTROLLING sediment entering
natural streams in irrigation return
flow is a major economic and
ecological challenge. A landowner
often has little control over the management
practices of his upstream
neighbors. But he may be able to use
sediment from runoff draining onto
his land to improve the land's topography.
In so doing he may reduce
erosion on his land and simultaneously
lower the downstream sediment
load

Carter, D.L.

Robbins, C.W.

USDA - ARS - NWISRL

• Ponds between cultivatedland a the Snake River• Pump hack systems pondsx Other ponds-- Natural drainsoh Line CawWagged by the U. S. UripartmunT of Agritunur .	 Conservation ofAP	 Ese Official Use	 sediment inirrigation runoffC. W. ROBBINS and D. L. CARTERCONTROLLING sediment entering natural streams in irrigation re-turn flow is a major economic andecological challenge. A landowneroften has little control over the man-agement practices of his upstreamneighbors. But he may be able to usesediment from runoff draining ontohis land to improve the land's topog-raphy. In so doing he may reduceerosion on his land and simultaneous-ly lower the downstream sedimentload.Sediment carried in irrigation returnflow usually is considered a pollutant,but small watersheds and drainage ba-sins contain many areas where sedi-ment can be removed from the waterand used as an economical source ofhigh quality topsoil for filling lowareas and leveling land. Small fieldscan be combined to form larger, moreeconomical farming units by fillingC. W. Robbins and D. L. Carter are soilscientists at the Snake River ConservationResearch Center, Agricultural Research Ser-vice, U. S. Department of Agriculture, Kim-berly, Idaho 8334/,gullies that divide them. Trappedsediment can also provide topsoil forlandscaping. Sediment trapped neardetachment sites for such uses is keptout of reservoirs, lakes, streams, andwater conveyance systems where itmight destroy aquatic habitats.An Idaho StudyThe 202,000 acres served by theTwin Falls Canal Company in southcentral Idaho contain about 150 nat-ural and man-made ponds that act assettling basins for irrigation runoff(Figure 1). This does not includevery small ponds that catch runoffwater from only one or two fields orponds used for municipal sewagetreatment or industrial waste disposal.We estimated the number of pondslarger than one-half acre from surveydata and classified them into threegroups. About 14 ponds were usedfor water storage and sediment re-moval for pumpback or sprinkler irri-gation systems. Another 42 naturaland man-made ponds, located betweenrow-rapped farmland and the canyonrim, served as settling basins for irri-Figure 1. Approximate location and number of sediment ponds on the Twin Falls Irriga-tion Tract In south central Idaho.Reprinted from We lemma] of WI and Water ConsenfetionHey•Juse 1175, Volume 30. thunber 3Copyright (r) 1175, gc.so•April 2, 19722	 A	 8	 1.082264882 CONCIPITRAT2811,0282212 IT WE128ITgallon return flow re-entering the 500-foot deep Snake River Canyon. Waterpassing through these ponds did notcross cultivated land other than pas-ture and was not likely to pick upmore sediment before it entered theriver. An estimated 87 ponds werescattered throughout the more inten-sively row-cropped portion of the irri-gation tract. These ponds, whichcatch sediment nearer the detachmentsource, were constructed for severalreasons.Some farmers constructed dikesacross gullies to trap sediment for fill-ing low areas, leveling land, and com-bining small fields into larger, moreeconomical units. Fills built acrossnatural gullies to support roads, rail-roads, or water conveyance systemsprovided ponds or pending potentialswhen the culverts were so designedor altered. A few ponds were con-structed to catch sediment to providetopsoil for landscaping around newbuildings or for placement back oneroded fields.On-farm ponds can trap consider-able amounts of sediment and keeptopsoil near the detachment source.In many cases, these practices can in-crease land values as much as $500 peracre.The nearly 150 ponds in the irriga-tion tract varied from new ponds builtafter the 1973 irrigation season toponds filled with sediment.Determining Sediment DispositionWe determined water flow and sed-iment deposition in a 1.1-acre pondspecifically constructed to trap sedi-ment in a low area near an irrigationreturn drain. The natural drain catchesirrigation runoff water from about 290cultivated acres. The basin supplyingwater to the drain is intensivelycropped to dry beans, sugarbeets,grain, alfalfa hay, and a limitedamount of pasture. Highly erodiblePortneuf silt loam covers the area.Slope varies from nearly 0 to about 15percent. The sediment was used tofill a low area that was wet during theirrigation season because the drainditch had been relocated along prop-erty boundaries at a higher elevation.A four-foot-wide dike was built alongthe two low sides, and water from thedrain was diverted through the pond.We determined trapped sedimentvolumes by surveying the newly con-structed pond in April 1972 beforeJuly 10, 1973Figure 2. Sediment pond at various stagesof maturity. Note striped stakes in pond.Figure 3. Sediment removed from irriga-tion runoff in relation to sediment concen-tration in runoff water.water was diverted into it, in October1972, and finally in July 1973 afterponding had been discontinued. Withthese volume figures and the bulkdensity of the deposited soil we calcu-lated sediment weight. A trapezoidalflume and water stage recorded at thepond outlet measured the water dis-charge. We collected water samplesduring the irrigation season and de-termined the sediment load in both in-flowing and outflowing water,Water was diverted into the pondon April 24, 1972 (Figure 2). By thefall of 1972 the pond was half full ofsediment. It filled completely by July5, 1973.During 1972, 167 acre-feet of waterpassed through the pond depositing1,254 tons of sediment. This was anaverage of 7.51 tons per acre-foot ofrunoff water or a yield of 8.4 acre-inches of topsoil (density of 1.3 ).Between April 29, 1973, and July8, 1973, 185 acre-feet of water passedthrough the pond depositing an addi-tional 1,379 tons of sediment or 7.45tons per acre-foot of runoff water.This amounted to 9.2 acre inches oftopsoil in 1973.The deepest point in the pond wasraised 38 inches with an average in-crease in elevation for the entire pondof 16 inches. This elevation increaseraised the level above the drain ditchand eliminated the swampy conditionthat existed previously.The pond removed 85 percent of thesediment in runoff water when thesediment concentration exceeded 0.1percent by weight (Figure 3 ).In addition to the land improve-ment benefits, sediment removal fromthe drainage water during the pond-ing period reduced the sediment en-tering the water distribution systemsand eventually the Snake River.A Temporary SolutionEffective sediment ponds can bebuilt in depressions or on shallow soilareas at minimum cost to a farmer ifhe uses his own equipment duringslack work periods. The sedmientcan be used to improve the land'stopography and value. Despite thesebenefits, however, trapping sedimentin man-made ponds must be consid-ered a short-term or emergencymeasure. Solution of the sedimentproblem over the long term dependson effective conservation treatment atthe sediment source.MAY-JUNE 1975	 135

Conservation and Use of Sediment in Irrigation Runoff

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Conservation and Use of Sediment in Irrigation Runoff

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