Dames & Moore, 1999 - USDA Forest Service

Reference Reaches - Stehekin River Watershed
Three stream reaches approximately 100 meters in length were selected within the Stehekin River watershed
as references for the other remaining downstream reaches of Railroad Creek. The selection of the reference
reaches was difficult due to the relatively unique conditions in Railroad Creek below the Site.
The reference reach aquatic sampling program also included the collection of surface water samples in order
to assess potential influences of water quality on the populations of benthic macroinvertebrates and fish.
Bridge Creek and South Fork of Agnes Creek as References for RC-5 and RC- 10
During the RI, it was discovered that the segment of Railroad Creek between RC-5 and RC-10 was unique
relative to the Railroad Creek upstream reference stations (RC-6 and RC- I), and with respect to gradient
(channel slope), watershed area size, and elevation. Consequently, it was decided to attempt to bracket the
RC-5 and RC- 10 conditions by selecting two reference reaches: one site on the South Fork of Agnes Creek,
which was smaller than the relevant stations on Railroad Creek, and one site on Bridge Creek, which was
larger than the relevant stations on Railroad Creek.
The Bridge Creek site was located approximately six miles upstream of the confluence of the Stehekin
River, at Sixmile Camp. The average total number of benthic macroinvertebrates collected in the eight
replicate samples were 997 per square meter. The actual values in the replicate samples ranged from 500
per square meter to 1820 per square meter. The total calculated number of fish caught at the Bridge Creek
site were 398 per hectare, predominantly cutthroat trout. The fish length ranged from approximately 10 to
250 millimeters. The majority of the fish at the Bridge Creek site were collected in one pool five to six feet
deep.
The South Fork of Agnes Creek site was located immediately downstream of the Swamp ~ieek
confluence. The average total number of benthic macroinvertebrates collected in the eight replicate
samples was 1058 per square meter. The actual values in the replicate samples ranged from 490 per
square meter to 1580 per square meter. The total calculated number of fish caught at the Bridge Creek
site were 67 per hectare, and a predominantly rainbow trout.
The South Fork Agnes Creek and Bridge Creek stations appear generally comparable to the RC-5 and RC-
10 stations in Railroad Creek. However, the majority of fish at the Bridge Creek station were caught in a
large pool. The presence of the pool. had a bearing on the numbers of fish caught. Since similar pools were
not present at the South Fork Agnes Creek, RC-5 and RC-10 stations, the fish data from the Bridge Creek
station are likely not comparable to these stations.
Benthic macroinvertebrate communities within RC-5a (52 organisms/m2) and RC- 10 (75 organisms/m2)
appear suppressed compared to the respective refFnce stations BC-1 and SFAC-I (997 and 1,058
organismslm2, respectively) based on the 1997 aquatic sampling results. The substrate throughout - both of
these stations contains a flocculent material that is readily suspended when disturbed. Surface water
samples collected from RC-7 during the 1997 investigation indicate that water quality criteria for acute
toxicity were exceeded during April (zinc), May (cadmium, copper and lead), and July (copper and zinc).
The reduced density of benthic macroinvertebrates may be the result of water quality conditions at RC-5a;
however, although the criteria were never exceeded at RC- 10. Benthic macroinvertebrate density at RC-
10 is similar to that of RC-5a. The reduced density of benthic macroinvertebrates coincident with the
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17693-005-01 9Uuly 28, 1999:10:24 AM;DRAFT FINAL RI REPORT