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ensure accuracy. wasteload allocations and total maximum daily
loads. The U.S. Environmental Protection Agency (EPA) also
supports the use of "biological" low-flow quantiles,
complementing the familiar statistical low-flow estimates
(USEPA, 1990, 1991). Estimation of biological low-flow quantiles
has been developed to reflect the continuous recovery periods
thought to be required by biological populations recovering from
low-flow stress. Heuristic techniques have been developed that
utilize the historical hydrologic record to identify flow levels
that would empirically achieve nominal frequency duration
criteria, based on historical streamflow records. Biological low
flows for acute, 1-hour/3-year (1B3) and chronic 4-day/ 3-year
(4B3) low-flow quantiles are recommended and have been utilized
by EPA (CFR, 1992) in setting toxic di