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The USFWS has therefore designated the median flow for August as the Aquatic Base Flow (ABF)., The USFWS has assumed that the ABF will be adequate throughout the year, unless additional flow releases are necessary for fish spawning and incuba- tion. We have determined that flow re- leases equivalent to historical median flows during the spawning and incubation pe- riods will protect critical reproductive functions. 'Aquatic Base Flow as used here should not be confused with hydrologic base flow, which usually refers to the minimum dis- charge over a specified period. The median August flow was selected by the USFWS as typical because essentially half of all the observed flow values would fall above it and below it and aquatic biota can, therefore, reasonably be expected to adapt to such a flow. Therefore, by the cri- teria established by the USFWS, the ap- propriate value to satisfy the New England ABF policy is the best approximation of historic, unregulated, median August flow. In cases where there are sufficient stream gage data on an unregulated river, the ac- tual historic median August flow can be calculated. However, where there are in- sufficient stream gage data or where the stream is regulated, the USFWS New En- gland Regional Office's policy is to pre- scribe art ABF of 0.5 cubic feet per second per square mile (cfsm) of drainage area as a preliminary recommendation. However, this policy further states that the USFWS may approve another instantaneous min- imum flow recommendation if the appli- cant or project sponsor provides adequate justification for the recommendation (USFWS 1981). These other recommenda- tions may involve either a habitat-based or a hydrologic-based analysis (G. Russell, personal communication). The recommendation of 0.5 cfsm ABF was derived by a synthesis of U.S. Geolog- ical Survey (USGS) gage data for selected New England streams by the USFWS New England Regional Office. The underlying premise of the 0.5 cfsm policy value is that the flow data records from unregulated streams can be used to approximate unreg- ulated flow in any New England stream. The objective of the USFWS at the time the policy was adopted was to establish a sin- gle ABF standard that could be used as a basis for site-specific recommendations anywhere in New England. In the absence of any other data, the 0.5 cfsm value has also been generally adopted as policy by most New England state aquatic resource agencies. The methodology supporting the 0.5 cfsm value was not reviewed technically prior to implementation of the policy by submitting the methodology to formal crit- ical review. There has been a growing con- cern among water users and resource man- agers that the 0.5 cfsm value may not be a universally applicable approximation of unregulated median August flow in all New England streams. As noted above, the USFWS has indicated in its policy that al- ternative methods of computing ABF's may be justified. Because New England is composed of distinct geomorphic and climatic regions, it follows that region-specific hydrologic patterns exist, and region-specific ABF val- ues may be more appropriate for establish- ing instream flow releases to maintain aquatic habitat than a single New England- wide value not pertinent to any particular region. The objective of this paper is to review and analyze the methodology used to de- rive the existing New England ABF rec- ommendation value of 0.5 cfsm and to rec- ommend an ABF method to protect aquatic resources subject to water diversion in var- ious parts of New England. This paper does not consider non-habitat resource protec- tion factors specific to particular river seg- ments that may influence flow require- ments, such as water quality and aesthetics. Two methods of calculating an August median flow (the USFWS 0.5 cfsm recom- mendation and the use of flow duration data) were compared, and an analysis of regional river basin characteristics and gage selection (including those used by USFWS in 1981) was conducted. This study was based on a literature search and review, analysis of USGS gage records and other pertinent data, and limited agency (USFWS and USGS) consultation regarding general statistical procedures. B. H. Kmlik 9 It