8142

• UCREFRP Catalog Number: 8142
• Author: Horn, M. J.
• Title: Nutritional Limitation of Recruitment in the Razorback Sucker ‎(
• USFW Year: 1996.
• Copyright Material: NO

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river's virgin flow now reaches the Gulf of California. As of 1995, the river provided

domestic water for more than 18 million people and irrigated 1.7 million acres of land in

the seven basin states in the U.S. and two states in Mexico (USDOI 1995).

A. Historic Conditions

Prior to human intervention and development the Colorado River flowed

unimpeded for - 2320 km to the Gulf of California from its head at >3105 m above mean

sea level in the Rocky Mountains (Figure 1) (Stanford and Ward 1986). The lowermost

part, from Grand Canyon downstream, had relatively low gradient, passing through long,

alluvial valleys alternating with shallow canyons in short, bedrock-confined reaches.

Channels in the valleys and on the extensive delta, formed braids, meanders, and isolated

oxbows in periods of low discharges. The floodplain was occupied by marshes, gallery

forests, and woodlands that were periodically altered or destroyed by violent, channel-

straightening floods (Kniffen 1931; Sykes 1937).

A highly variable hydro graph associated with tremendous sediment loads

dominated both physical and biological attributes of this system. Over the long term,

spectacular changes in discharge were the rule. Recorded flows at Yuma, AZ, between

1925 and 1935 varied from 18 to 7,500 m3/sec (USGS 1973). On an average hydro graph,

highest annual discharges in late April through July reflected snowmelt from the Rocky

Mountains, although spring time flows started increasing in the lower river in February

and March in many years (USGS flow data). Lower flow characterized the