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versions existed by the late 1800's, and con- struction of large mainstream projects be- gan in the early 1900's (Fradkin 1981; Carlson and Muth 1989). The Bureau of Reclamation initiated its major dam construction phase in the Col- orado River with passage of the Colorado River Compact and Boulder Canyon Pro- ject Act in 1928 and 1929 (Fradkin 1981). By the 1960's, much ofthe mainstream Col- orado River had been converted into a sys- tem of dams and diversions. As a result, the timing, duration, and magnitude of flows of most mainstream rivers of the Col- orado River basin have been substantially altered. This includes downstream changes in flow, temperature, and channel mor- phology; inundation of stream habitats; and habitat fragmentation due to stream block- age (Miller 1961; Ono et al. 1983; Carlson and Muth 1989). Habitat change in the Colorado River has been associated with the proliferation of nonnative forms introduced by man and the decline of native species (Minck ley 1982; Carlson and Muth 1989). The dis- appearance of native fishes from greatly altered habitats indicates that habitat change, including invasion by other spe- cies, has occurred too quickly for native forms to adapt and recover (Minckley and Deacon 1968; Molles 1980). It is more dif- ficult to determine causes for declines of endangered fish stocks that occupy less- altered habitats. Changes in riverine habitats and endan- germent of native fishes has been of con- cern to the U.S. Fish and Wildlife Service (FWS). The FWS has consulted with other federal agencies in the upper Colorado River basin under provisions of the En- dangered Species Act of 1973 as amended (16 U.s.e. ~ 1531 et seq.), and has issued more than 100 Biological Opinions pur- suant to Section 7 of that Act (Rose and Hamill 1988). In general, the FWS deter- mined that water depletion and dam op- eration would likely jeopardize the endan- gered fishes. However, FWS opinions on flows that are required by the fish may conflict with water rights of various states as well as Interstate Compacts and other agreements (Rose and Hamill 1988; Wy- doski and Hamill 1991). An interagency Recovery Implementation Program was es- tablished in the upper Colorado River ba- sin in an effort to recover the listed fishes to a nonendangered status, and to seek ways to provide streamflows for them under ex- isting state and federal water agreements (Wydoski and Hamill 1991). The need for annual flow regimes for the endangered fishes in the upper Colorado River basin has been identified by the Re- covery Implementation Program, but var- ious cooperators disagree on the methods used to determine their magnitude, tim- ing, and duration (unpublished reports on file at the FWS office in Denver, Colorado). Management and recovery of endangered fishes is a relatively new concept, and re- covery methods are not well developed (Williams et al. 1989). Future management of endangered Colorado River fishes will require the development of recovery goals in which instream flow needs are clearly identified, and innovative approaches will be required to meet the flow needs of the fish within water constraints. The objectives of this article are to: (1) report the status of the endangered, big- river Colorado River fishes, (2) contrast two philosophies guiding the determination and implementation of in stream flows, and (3) recommend an approach for determin- ing instream flow needs of endangered Colorado River fishes that will promote their recovery and lessen the likelihood for decline of other native fishes. STATUS OF COLORADO RIVER FISHES A long period of geographic isolation and extreme climatic and hydrologic con- ditions have resulted in a unique and in- sular Colorado River fish fauna (Miller 1959,1961; Molles 1980). This fauna can be separated into three categories: (1) fishes inhabiting high or intermediate elevations that either share, or have closely allied ~ 28 forms in, adjacent drainages; (2) species en- demic to small streams at low to interme- diate elevations; and (3) big-river fishes, commonly called the Colorado River fish- es, that are mostly species endemic to mainstream rivers (Minckley et al. 1986). Native big-river fishes, consisting of cyp- rinids (minnows) and catostomids (suck- Rivers . Volume 3, Number 1 January 1992