Floyd A. Nagler

The Water Yield from Small Water Sheds in Iowa

With the ever increasing pollution of our middle western streams by industrial and city sewage, it is becoming more difficult for towns situated in the river valleys to secure safe and usable water. It is only a matter of time until many Iowa municipalities will be forced to seek their water supply from the flow of small streams whose water sheds can be protected from all dangerous pollution, which is now the prevailing practice in New England. Even though the yield of Iowa streams is less than those in the East, the runoff from exceedingly small water sheds would prove adequate for the municipal supply of most of our Iowa towns. In some parts of Iowa this flow could be conducted by gravity to the municipality concerned, but, in many parts of the state, pumping would be required. With adequate storage, the average watershed may be expected to yield close to 300,000 gallons per day per square mile. In years of minimum flow, the stream may yield only onethird of this amount. But it is not a safe procedure to base a final estimate of the yield from a small stream upon the average behavior of its neighbors. More than all other types, small streams are deceptive. Unfortunately, very few records have been maintained from which the yield of small midwestern streams may be accurately determined. It is impossible to estimate their behavior from that of similar water sheds located either east or west of Iowa, even though both receive the same amount of precipitation. Stream flow observations have been maintained during portions of the past twenty-five years upon many of the larger streams of Iowa. A study of these records reveals a big difference in the average water yield per square mile of the larger streams within the state. If such differences exist in the water yield of the larger