9508

• UCREFRP Catalog Number: 9508
• Author: Valdez, R. A. and P. Nelson.
• Title: Green River Subbasin Floodplain Management Plan.
• USFW Year: 2004.
• USFW - Doc Type: Lakewood, Colorado.
• Copyright Material: NO

3.0 Scientific Basis And Underlying Principles

3-10

April 2004

Studies of Green River floodplains indicate that razorback sucker can survive in the
presence of large numbers of nonnative fishes following a year of desiccation. In October, 1995,
Modde (1997) reported 28 age-O razorback sucker (3.7 inches TL, 94 mm TL) in Old Charlie
Wash in the presence of large numbers of nonnative fishes and after a dry period in 1994. In
August, 1996, Modde (1997) also reported 45 age-O razorback sucker (2.6 inches TL, 66 mm
TL). Assuming that these fish entered Old Charlie Wash as larvae during runoffin the previous
June, the fish captured in October, 1995, were about 4 months old, and those captured in August,
1996, were about 2 months old. The Floodplain Model predicts highest growth rate of razorback
sucker at 94 mm TL in about 3.3 months, and growth to 66 mm TL in about 2.3 months. Hence,
growth exhibited by these wild fish in the presence of large numbers of nonnatives was higher
than or equal to highest growth rates for the species. Survival rate of these fish was not
determined because the initial numbers of entrained young was not known.

A separate study tested the hypothesis that larval razorback sucker can survive in
floodplain depressions following a reset year (Birchell and Christopherson 2002). Larval
razorback sucker and bonytail stocked into the Stirrup floodplain in May 2002 in the presence of
adult fathead minnow, red shiner, black bullhead, green sunfish, and common carp survived at
rates of 1.7-1.9% for bonytail (17.1% in control) and 0.4-0.7% for razorback sucker (12.0% in
control). A study to evaluate the Leota floodplain as a grow-out site assessed survival of 66,110
stocked larvae and 900 razorback sucker of various sizes during March through May 2001. A
total of 84 razorback sucker were recaptured, including 35 age-O in the presence of large numbers
of nonnative fishes. Specific survival rate could not be determined because fish could have
escaped during draining ofthis floodplain site.

3.7 Floodplain Management Strategy

The recommended management strategy for floodplains of the upper basin is based on the
"reset theory" of inundation and desiccation of depressions on a 12 or 24-month cycle. The
"reset theory" of floodplain management has not been implemented and tested in its entirety.
Components of the strategy have been successful as described in this Plan, and uncertainties,
risks, contingencies are presented in section 7.5. This strategy is illustrated in Figures 3-4 and 3-
5. The success of this floodplain management strategy depends on six factors:

1. Connection of the floodplain with the river channel in year 1;
2. Entrainment of drifting larvae in year 1;
3. Sufficient food production with a chronology of development timed to arrival of
larvae;
4. Suitable quantity and quality of water to support fish for 12 or 24 months;
5. Reconnection of the floodplain in year 2 or 3 to allow escapement offish to the
mainstem and for freshening of water quality in the floodplain; and
6. Periodic desiccation to reset floodplain.