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2.2 - Susitna River Water Quantity
2.1.2 - Sloughs
Sloughsare spring-fed, overflow channels that exist along the
edge of the floodplain, separated from the river by well-vegetated
bars. An exposed all uvi al berm often separates the head of
the sloughs from the mainstem or side-channel flow. The controlling
streambank elevations at the upstream end of the sloughs are
less than the mainstem water surface elevations during median and
high flow periods. At intermediate and low flows, the sloughs
convey clear water from small tributaries and upwelling ground
water (ADF&G 1982a).
Differences between mainstem water surface elevations and the
streambed elevation of the sloughs are notably greater at the
upstream entrance to the sloughs than at the mouth of the
sloughs. The gradients within the sloughs are typically greater
than the adjacent mainstem because of their shorter path length
from the upstream end to the downstream end, than along the rnai nstem.
The upstream end of the sloughs generally has a higher
gradient than the lower .end. This is evidenced ;In Figure E.2.21,
which illustrates the thalweg profile of a typical slough.
The sloughs vary in length from 2,000 to 6,000 feet (610 to
1829 m). Cross-sections of sloughs are typically rectangular
with fl at bottoms as ill ustrated in Fi gure E.2.22. At the head
of the sloughs, substrates are dominated by boulders and cobbles
[8 to 14 inch (20 to 36 cm) diameter]. Progressing downstream
towards the slough mouth, substrate particles reduce in size with
gravel s and sands predomi nating (Figure E.2.21). Beavers frequently
inhabit the sloughs. Active and abandoned dams are
visible. Vegetation commonly covers the banks to the water's
edge with bank cutting and slumping occurring during spring
break-up flows and high summer flows.
The importance of the sloughs as salmon spawning habitat is discussed
in detail in Chapter 3.
2.2 - Susitna River Water Quantity
2.2.1 - Mean Monthly and Annual Flows
Continuous historical streamflow records of various record length
(7 to 32 years through water year (WY) 1981) exist for gaging
stations on the Susitna River and its tributaries. USGS gages
are located at Denali, Cantwell (Vee Canyon), Gold Creek and
Susitna Station on the Susitna River; on the Maclaren River near
Paxson; at Chulitna Station on the Chulitna River; at Talkeetna
on the Talkeetna River; and at Skwentna on the Skwentna Ri ver.
E-2-10