Dames & Moore, 1999 - USDA Forest Service

the stream substrate (Figure 4.3-3). However, iron staining was observed as coatings on the stream substrate
throughout this stream segment. A more in-depth discussion of the iron-oxide precipitate and ferricrete is
presented in Section 4.3.9.
Tenmile Creek to Lake Chelan
Less than approximately 500 feet downstream of the confluence of Tenmile Creek (approximately one-half
mile downstream of the Site), a large log jam is present which causes a flattening of the stream gradient to
less than one percent. This appears to be the flattest channel gradient in the portion of the basin downstream
of the wilderness boundary, and minor accumulations of fine sediments were observed on the streambed in
backwater areas. Also in this area, beaver activity is evident, wetlands occur, and .the channel and
floodplain broaden.
Downstream of the log jam below Tenmile Creek, the channel gradient gradually increases. Bar
development and braiding in the channel decreases and bedrock outcrops at some locations. At
approximately river mile 6.5 (as measured from the mouth and approximately 4 miles downstream of the
Site), a large erosional feature occurs, which consists of a 50-foot-wide dry cobble and boulder channel that
appears to have been recently carved by Railroad Creek. The channel, consisting of a vertical cutbank of 20
or more feet on the left (north) bank occurs where a recent avalanche entered, the channel.from the south
(right) valley slope, depositing debris on the opposite bank. The eroded channel appears to have been
caused by the avalanche debris blocking the channel.
Downstream of this erosional feature, the channel gradient increases to greater than 5 percent. The average
substrate size, likewise, generally increases from predominantly cobble- to boulder-size particles in a
downstream direction. The channel also becomes, increasingly confined to a steep sided valley, with little
floodplain development. Iron staining also diminishes in this reach. Iron staining on the streambed substrate
becomes nearly indistinguishable from algae and other coatings on the rocks below approximate river mile
5.0 (approximately 5-112 miles downstream of the Site). Bedrock outcrops and channel control increase
between river mile 5.0 to river mile 3.0, where the channel enters a steep walled bedrock canyon, with
several large waterfalls. Below the canyon, the gradient gradually decreases until the creek enters Lake
Chelan; however, bed substrate remains large, dominated by cobbles and small boulders.
Stream flow
- Site
Streamflow in Railroad Creek exhibits a typical snowmelt dominated hydrograph, with sustained peak flows
generally occurring in spring and low flows following through the fall and winter seasons. Large rain events
can cause short lived peak events anytime during the summer and fall, and can contribute to peak flows in
the spring. Baseflows are sustained by groundwater and upper basin meltwater, including glacial melt, in the
summer. During the fall and winter months, groundwater sustains the baseflow, as melting of snow ceases
when temperatures drop below freezing. During the September 1997 RI, flow in Railroad Creek adjacent to
the Site was observed to decrease rapidly (commonly less than one day) when temperatures in the upper
elevations appeared to fall below freezing based on observed snow accumulation, indicating the effect of
freezing on the reduction of melt flow contribution to baseflow.
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17693405-019Uuly 19. 1999;4:51 PM;DRAR FINAL RI REPORT