BOARD02098

• Board Meeting Date: 11/23/1998
• Description: WSP Section - Colorado River Basin Issues - San Juan Recovery Program - Status Report
• Board Meetings - Doc Type: Memo

Mimicry of the natural hydrograph is the foundation of the flow recommendation process for the San
Juan River. The flow recommendations require mimicry of statistical parameters of flow based on
flow/geomorphologylhabitat linkages and the statistical variability of the pre-dam hydrology rather
than mimicry of each annual hydrograph. Therefore, the resulting flows will not mimic a natural
hydrograph in all years, but will mimic the variation and dynamic nature of the 65-year record of the
San Juan River.

The hydrograph recommendations are designed to meet the conditions required to develop and
maintain habitat for Colorado squawfish and razorback sucker and provide the necessary hydrologic
conditions for the various .life stages of the endangered and other native fishes. The conditions are
listed in terms of flow magnitude, duration (days at or above specified magnitude), and frequency
(average recurrence of the conditions specified, expressed as a percent and a maximum allowable
duration of years without meeting the condition). To allow for gage and modeling error and the
difference between the flows at the historical gage at Bluff, Utah, and the Four Corners gage,
maximum allowable durations are computed for 97% of the target flow rate. In most cases, the
primary recommendation is for a specified flow rate (I.e., 10,000 cubic feet per second (cfs)) of a
minimum duration (I.e., 5 days) for a specific frequency of occurrence (I.e., 20% of the years). In
addition to the primary recommendation, variability in duration is desirable to mimic a natural
hydrograph. Therefore, a frequency table for a range of durations for each flow rate is
recommended. A maximum duration between occurrences is also specified to avoid long periods
when conditions are not met, since such long periods could be detrimental to the recovery of the
species. The maximum period without reaching a specified condition was determined as twice the
average required interval (except for the 80% recurrence of the 2,500 cfs condition, where 2 years
is used). For example, if the average interval is 1 year in 3, then the maximum period between
meeting conditions would be 6 years. The maximum periods were based on an assessment of the
maximum time possible between conditions before substantial or irreversible impacts to the fish or
their habitat resulted. Following are the conditions specified:

A.

Category:

Flows> 10,000 cfs during tunoffperiod (March 1 to July 31).

Duration:

5 days minimum.

Frequency:

20% on average. Maximum period without meeting at least 97% of the
specified conditions is 10 years.

Purpose:

Flows above 10,000 cfs provide significant out-of-bank flow, generate new
cobble sources, change channel configuration providing for channel diversiry,
and provide nutrient loading to the system, thus improving habitat
productivity. Such flows provide material to develop spawning habitat and
maintain channel diversity and habitat complexity necessary for all life stages
of the endangered fish. The frequency and duration are based on mimicry of
the natural hydro graph, which. is important for Colorado squawfish
reproductive success, and maintenance of channel complexity, as evidenced

SJRIP Biology Committee
16 September 1998

8-5

Executive Summary
Draft Aow Report

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