The low-flow characteristics of a stream largely govern the type and economics of its utilization. The magnitude, duration, and frequency of low flows are used to determine if a water-utilization project can be operated without storage or to compute the amount of storage required for its operation. The frequency of low flows affects the economics of both construction and operation of a water-utilization project.The character and distribution of geologic formations within stream basins influence the quality and quantity of the low flows of streams. When direct runoff from precipitation ceases, the flow of streams is governed by the volume of water in ground storage and by the rate at which the ground water discharges into the stream.Manmade changes to the land and to the stream systems probably have altered the regimen of flow of many streams.Limited low-flow data, in cubic feet per second per square mile, for 32 daily-record gaging stations and 57 partial-record stations, are summarized for ready comparison. The summary gives the minimum average 7-day and 30-day discharges that may be expected to recur at 2-year arid 10-year intervals and gives the flow at the 95-and 90-percent duration points. More detailed data on the magnitude and frequency of low flows and flow duration, in cubic feet per second, are given for the 32 daily-record gaging stations.The 7-day low flow at the 2-year recurrence interval expressed on a per-square-mile basis is used to demonstrate areal variations of low flow in the study area. These indices range from 0 to 0.61 cubic foot per second per square mile.Streams north of a low ridge that extends across the area north of the North Fork Obion River near the Tennessee-Kentucky line generally have relatively low indices of base flow, whereas streams immediately south of the ridge generally have higher indices of base flow. The reason for these marked differences in low-flow indices is not known, but displacements resulting from recent crustal movements in the area may affect the movement of ground water toward the streams. Ground water from the higher tract in Kentucky may drain toward the streams in Tennessee as underground flow.Streams along the eastern margin of the embayment in Kentucky receive their base flow from the Cretaceous formations and Pliocene (?) deposits, and streams in the remainder of the State receive their base flow from the Eocene formations. Most headwater streams in Kentucky are above the groundwater table and are intermittent. At some downstream point the channels intersect the water table in the Paleozoic, Cretaceous, and Eocene deposits, and downstream from the point of intersection the streams are perennial. In some places, however, the amount of surface flow is lessened because of the amount of water moving out of the valley as underflow.Streams north of the Ohio River in Illinois, and those in the embayment where the interstream areas are mantled by the relatively impervious loess, have fairly low indices of base flow.Streams receiving their base flow from the "50...

The Mississippi embayment is part of a vast geologic and hydrologic province. Most of the region is underlain by aquifers that will yield large quantities of water to wells, so that ground water is the most readily available source of fresh water, Ground water having a dissolved-solids content of less than 500 ppm (parts per million) is generally available at depths of less than 1,000 feet, and water having a dissolved-solids content of less than 1,000 ppm is available in some places to depths of more than 2,000 feet. Iron is the most common troublesome chemical constituent in the ground water. The potential yield of the aquifers that underlie the region is estimated to be about 30,000 mgd (million gallons per day), of which about 3,000 mgd is presently being withdrawn. Water in varying amounts is also available from streams within the region. The amount of water which originates within the region and which leaves it as streamflow during a year averages about 90 million acre-feet (about 80,000 mgd). An additional 400 million acre-feet (about 360,000 mgd) leaves the region as streamflow during an average year, this amount having originated outside the region. The present withdrawals (1965) from streams within the region are about 1,700 mgd.

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_________ Manmade changes Tombigbee-Black Warrior River basin. Pearl River basin___________________ Yazoo River basin__________._______ Big Black River basin________________ Farm ponds ar>d lakes.-______________ Low-flow characteristics__-___-_________. Low-flow frequency._____________ _. Flow duration.______________________ Factors affecting low flow_______________ Eastern Gulf of Mexico basins. Tombigbee-Black Warrior River basin.

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