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68 relative to the basins, slow enough for the river to cut the canyons which still exist today. No evidence of ponding r lakes along the mainstream south of Needles (River Mile 245) exists, but major wash systems draining adjacent mountain ranges and channel braiding in the river valleys were common. A series of two aggradations and degradations have occurred with the last aggradations be-inning 10,000-15,000 years ago. During these changes in riverbed elevation, the slope stayed about the same near 1.7 ft drop per linear mile of river. The river character changed dramatically though, from a system moving pebble- to cobble-sized gravel to one moving a median-particle size of 44 microns which is silt (Metzger et al., 1973). High flow events, generally peaking May-July, created new gravel bars and renovated old ones annually. The area of gravel substrate was primarily upstream of Parker, Arizona with the exception of wash fans from lateral drainage areas. Similar bar development occurred in major tributaries, only on a smaller scale. The literature indicates that razorback sucker once migrated considerable dis- tances and concentrated in large numbers near the mouths and up into the tributaries, presumably to spawn (Jordan, 1891; Hubbs & Miller, 1953; Sigler & Miller, 1963). Recent information indicates specific substrates are selected for spawning (Linda Ulmer, pers. comm.) approximating substrates similar to historic bars or wash fans. The confluence areas of major tributaries and the mainstream (wash fans included) may have provided the loose, clean substrate during the late or post runoff periods on the Colorado River. Gravel on newly formed or active bars would have good percola- tion through interstices and spawning adults could move substrate easily during the deposition of gametes. During the period 1909-1938, seven dams were built on the lower Colorado River which has resulted in colder, but more uniform water temperatures extending longer into the spring, and the warming trend into the summer (post runoff) is slower. Peak flows are now later into the summer and there is an inverse relationship between flow and temperature. As flows increase to meet irrigation/power demands temperatures are cooler and peak flows are not adequate to renovate or create bars. Migration routes to the majority of tributary streams in the upper and lower basin have been blocked. Reservoirs along the system where remnant sucker populations exist are aging, substrate is silted in and compacted, and temperature regimes are variable. Channel- ization in river segments between dams has reduced backwaters and potential spawning and rearing habitats. Spawning observations in 1980 and 1981 in Senator Wash Reser- voir and Lake Mohave were over lap-zone gravel areas (Linda Ulmer pers. comm.; W. L. Minckley, pers. comm.) which are continually cleaned of silt by wave action. Only in the Yampa River (Colorado) is the channel substrate and temperature near natural conditions and now headwater storage projects in that drainage are proposed (R. Valdez & E. Wicks, pers. comm.). It was estimated that razorback sucker range was limited to elevations near or below 5,000 ft in Colorado (Ellis, 1914), and that only a limited amount of stream habitat there was below that elevation, and the majority of the river downstream has been altered. LIFE HISTORY REVIEW Maturity in razorback sucker (X. texanus) appears to begin around 5-7 years of age (Tel. L. Minckley, pers. comm.; L. Courtois, pers. comm.) and spawning occurs at temperatures 12-180C in January through April depending on the weather, water operations and location along the system. Spawning depths from 0.5-3.0 meters are most common. Dams now block migration and now only specific locations provide spawning substrate in the lower river'(Loudermilk, 1982, in press). Only a small per- centage of gonad contents are released during each spawning act, and there are