7758

• UCREFRP Catalog Number: 7758
• Author: Stanford, J. A. and P. C. Nelson.
• Title: Instream Flows to Assist the Recovery of Endangered Fishes of the Upper Colorado River Basin.
• USFW Year: 1994.
• USFW - Doc Type: Denver, Colorado.
• Copyright Material: NO

INSTREAM FLOWS TO ASSIST THE RECOVERY OF ENDANGERED FISHES 25
zoobenthos communities are more productive
on cobble bars than sand, but substratum size
on river bars is highly variable as a function of
the dynamic sediment transport and deposi-
tion processes that occur as the river fluctuates
between peak and base flows (Fig. 2). Another
example, though not well documented, is the
propensity for high benthic and planktonic pro-
duction in subchannels (backwaters) and flood-
plain wetlands that were (predam) seasonally
flooded. These different, yet interactive, space
and time scales that produce natural biophysi-
cal variation are the essence of the ecosystem
in which the endangered fishes evolved and
must be documented thoroughly.
4. Studies in the Upper Colorado River Basin
indicate that flow regulation, specifically re-
duction of the amplitude between peak and
base flows, is a likely contributor to the decline
of the native fishes, but the cause and effect
relationship is not simple. For example, years
of regulated flows, coupled with construction of
revetments, seem to have reduced the avail-
ability of backwaters and wetlands as nursery
habitats that support larval and juvenile
squawfish. Although extremely high flows
seem to be associated with weak cohorts of
Colorado River squawfish and humpback chub,
occasional extreme flooding needed to main-
tain channel morphology and channel-flood-
plain interactions probably is critical for long-
term survival of the fishes. Indeed, the only
recent incident of successful recruitment of
adult razorback sucker occurred when high
flows reconnected riparian gravel pits to the
mainstem Colorado River. On the other hand,
squawfish recruitment can occur over a wide
range of spring flows, and squawfish spawning
may be much less site-specific than is indicated
by the literature, or a wide range of preferred
spawning conditions exists on the spawning
bars where squawfish are routinely found (e.g.,
Cleopatra's couch bar on the Yampa, Three
Fords on the Green). Presence of nonnative
predators and reduced complexity of habitats
needed by the different life history stages of the
endangered fishes (due to severing of channel-
floodplain connections and encroachment of ri-
parian vegetation into the channel) further
confound determination of cause and effect.
The fundamental problem with respect to pro-
vision of flows to recover the endangered fishes
is balancing the many interactive effects in a
manner that will favor the native fishes over
the long term (i.e., decades).
5. The life histories of the endangered fishes, as
well as those of zoobenthos that also have been
studied in detail, are either directly or indi-
rectly controlled by flow magnitude and timing
and the relation between flow and tempera-
ture. However, relationships between flow,
channel configuration, and thermal heteroge-
neity (cf., Ward 1984) have not been well inte-
grated conceptually or empirically or in the
context of the various life history stages of the
fishes. A squawfish life history energetics
model, for example, would be very helpful in
this regard.
6. Stream regulation has introduced serial discon-
tinuities (i.e., downstream extension of coldwa-
ter or rhithron environments) within the river
continua of the Upper Colorado River Basin.
The location and persistence of these disconti-
nuities are directly related to flow and largely
determine where the endangered and other
native fishes can achieve a positive life history
energy balance (i.e., complete the life history
with net recruitment of young at or above mini-
mum viable population size). Remember, these
fishes are adapted to potamon conditions, and
the length of the potamon zone has decreased
as a consequence of the downstream extension
(discontinuity) of the rhithron zone through
regulation of flow from the deep storage reser-
voirs. The concept of ecosystem "resets" and
discontinuities (sensu Ward and Stanford
1983), coupled with the notion that connected
channel and floodplain (backwaters, wetlands)
components of the riverscape are seasonally
pulsed by flooding (Ward 1989), robustly inte-
grates the myriad biophysical processes that
are influenced by stream regulation. Strong
inferences about how a river ecosystem may
respond to alternative flow management ac-
tions must be derived in this ecosystem con-
text. The downstream shift in the position of
the rhithron-potamon transition is an ecosys-
tem-level measure of change wrought by regu-
lation and should be used to adjust flows to
maximize conditions known to be favorable to
potamon (e.g., endangered fishes) and rhithron
(e.g., trout) fisheries.
7. Strong food web interactions are probably oc-
curring as a consequence of the presence
of a wide variety of nonnative fishes, which
now dominate fish communities throughout