Hydrology of 7-Day 10-Yr Low Flows

The level of treatment necessary for maintaining stream quality depends on the dilution ratio, defined as the ratio of the 7-day 10-yr low flow in the stream to the average dry weather flow from the treatment plant. The natural low flows depend on physiography, baseflow regime, stream entrenchment, flood-plain development, magnitude of the drainage area of the stream, etc. The natural low flows are modified by recreational pools created by in-river impoundments, reservoirs or lakes for recreation and regulation of flows, urbanization, municipal and industrial water use and wastewater treatment plant effluents, and navigation pools. Downstream of the confluence of two major streams draining hydrologically dissimilar areas, the low flow exceeds the sum of the two individual low flows, because they are out of phase with each other. Examples are presented showing the effect of these factors on the 7-day 10-yr low flow in streams.