| Klahr, Patricia C.; 1988; Lake Irrigation District Survey and Cascade Reservoir Tributary Assessment, Valley County, Idaho 1986; Water Quality Status Report No. 79; Idaho Department of Health and Welfare, Division of Environmental Quality, Water Quality Bureau, Boise, Idaho; 46p.
An evaluation of the impact of irrigation practices on surface water quality in the Cascade Reservoir watershed conducted during 1986. Monitoring stations were located above and below irrigated crop and pasture land on irrigation ditches. Samples for nutrients, sediment and bacteria were collected every two weeks during the irrigation season. Flood irrigated cropland showed the highest water quality impacts (dissolved ortho-phosphate increased over 600% and total nitrogen increased over 180%), sprinkler irrigated cropland showed the least amount of change. No correlation was noted with the absence or presence of grazing
animals. Irrigation return flows from flood irrigated crop and pasture lands exhibited total
phosphorus levels above 0.05 mg/L (the level at which accelerated eutrophication occurs) 100% of the time. Irrigation return water from sprinkler irrigated cropland exceeded this level about 50% of the time. Boulder Creek consistently exhibited the highest levels of total phosphorus, dissolved ortho-phosphorus, and bacteria. Individual tributaries showed differences in sediment and phosphorus loading, but contributed significantly more phosphorus to the reservoir than the McCall sewage treatment plant during spring runoff. The treatment plant was shown to contribute more phosphorus to the reservoir than the tributaries however, during low flow periods in late summer. Average concentration of total phosphorus in the Cascade Reservoir watershed (-0. 10 mg/L) was five to ten times the amount expected in an uncontaminated watershed, but may be due in part to inadequate detection limits in analytical methodology used. Conversion from flood to sprinkler irrigation for cropland, filter strips, sedimentation basins, contour plowing, and reduction of McCall sewage treatment effluent are recommended as applicable best management practices. A fairly extensive literature review is included. |
