before the ferc, application - Alaska Resources Library

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2 - BASELINE DESCRIPTION
The entire drainage area of the Susitna River is about 19,400 square
miles, of which the drainage area above Gold Creek comprises approximately
6160 square miles (Figure E.2.1). Three glaciers in the Alaska
Range feed forks of the Susitna River, which flow southward for about
18 miles (30 km) and then join to form the Susitna River. The river
flows an additional 55 miles (90 km) southward through a broad valley
where much of the coarse sediment from the glaciers settles out. The
ri ver then f1 ows westward about 96 mn es (154 km) through a narrow
valley, with constrictions at the Devil Creek and Devil Canyon areas
creating violent rapids. Numerous small, steep gradient, clear-water
tributaries flow into the Susitna in this reach of the river. Several
of these tributari es cascade over waterfall s as they enter the gorge.
As the Susitna curves south past Gold Creek, 13 miles (21 km) downstream
from the mouth of Devil Canyon, its gradient gradually
decreases. The river is joined about 40 miles (64 km) beyond Gold
Creek in the vicinity of Talkeetna by two major tributaries, the
Chul itna and Talkeetna Rivers. From this confluence, the Susitna flows
south through braided channels for 97 miles (156 km) until it empties
into Cook Inlet near Anchorage, approximately 318 miles (512 km) from
its sou rce.
For ease of discussion, the watershed has been divided into three
drainage basins. The upper drainage basin extends from the glacial
headwaters of the Susitna River to the confluence of the Tyone River.
The middle basin extends downstream from this point to Talkeetna and
contains the Watana and Devil Canyon damsites. The middle reach is
where the major project-related impacts will occur. The lower basin is
defined as the drainage basin from Talkeetna to Cook Inlet. The
approximate boundaries of the three basins are shown in Figure E.2.1.
The Susitna River is typical of unregulated northern glacial rivers
with high, turbid summer flow and low, clear winter flow. Runoff from
snow melt and rainfall in the spring causes a rapid increase in flow in
May from the low discharges experienced throughout the winter. Peak
annual floods usually occur in June.
Associated with the higher spring flows is a 100 fold increase in sediment
transport which persists throughout the summer. Between June and
September, the large suspended sediment concentration causes the river
to be highly turbid. Glacial silt, released by the glaciers when they
begin to melt in late spring or re-entra"ined from the river banks by
high flows, is responsible for much of the turbidity.
Rainfall-related floods often occur in August and early September, but
generally these floods are not as severe as the spring (May-June) snowmelt
f1 oods.
As the weather begins to cool in the fall, the glacial melt rate decreases
and the flow in the river correspondingly decreases. Because
most of the suspended sediment is caused by glacial outwash, the
E-2-3