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In cooperation with the Conodoguinet Creek Watershed Association

Effects of Removing Good Hope Mill Dam on Selected Physical, Chemical, and Biological Characteristics of Conodoguinet Creek, Cumberland County, Pennsylvania

U.S. Geological Survey Scientific Investigations Report 2005-5226

By Jeffrey J. Chaplin, Robin A. Brightbill, and Michael D. Bilger

This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.

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The implications of dam removal on channel characteris­tics, water quality, benthic invertebrates, and fish are not well understood because of the small number of removals that have been studied. Comprehensive studies that document the effects of dam removal are just beginning to be published, but most research has focused on larger dams or on the response of a sin­gle variable (such as benthic invertebrates). This report, pre­pared in cooperation with the Conodoguinet Creek Watershed Association, provides an evaluation of how channel morphol­ogy, bed-particle-size distribution, water quality, benthic inver­tebrates, fish, and aquatic habitat responded after removal of Good Hope Mill Dam (a small “run of the river” dam) from Conodoguinet Creek in Cumberland County, Pa.

Good Hope Mill Dam was a 6-foot high, 220-foot wide concrete structure demolished and removed over a 3-day period beginning with the initial breach on November 2, 2001, at 10:00 a.m. eastern standard time. To isolate the effects of dam removal, data were collected before and after dam removal at five monitoring stations and over selected reaches upstream, within, and downstream of the impoundment. Stations 1, 2, and 5 were at free-flowing control locations 4.9 miles upstream, 2.5 miles upstream, and 5 miles downstream of the dam, respec­tively. Stations 3 and 4 were located where the largest responses were anticipated, 115 feet upstream and 126 feet downstream of the dam, respectively

Good Hope Mill Dam was not an effective barrier to sedi­ment transport. Less than 3 inches of sediment in the silt/clay-size range (less than 0.062 millimeters) coated bedrock within the 7,160-foot (1.4-mile) impoundment. The bedrock within the impoundment was not incised during or after dam removal, and the limited sediment supply resulted in no measurable change in the thalweg elevation downstream of the dam. The cross-sec­tional areas at stations 3 and 4, measured 17 days and 23 months after dam removal, were within 3 percent of the area measured before removal.

Some of the impounded silt/clay at station 3 and other sed­iment in the work area downstream of the dam were initially entrained over the 3-day removal period and deposited on sub­strate at station 4. Remaining silt/clay at station 3 and deposits at station 4 were transported downstream by the flows mea­sured over the 23 months after removal (daily mean flow ranged from 38 to 5,180 cubic feet per second). The median bed-parti­cle size at station 3 increased by approximately 32 millimeters in the 23-month period after removal. Bed-particle-size distri­bution at station 4 became finer when silt/clay was initially deposited but coarsened as high flows flushed it downstream; median bed-particle size was 77.7 millimeters before removal compared to 31.3 millimeters 17 days after removal and 99 mil­limeters 23 months after removal.

Good Hope Mill Dam had either no effect on water-quality characteristics or the effect was so small it was masked by sea­sonal and periodic variability. Measurements of daily mean temperature, dissolved-oxygen concentration, pH, and specific conductance on a short time scale (every 15 minutes) indicate the daily range of temperature was suppressed under impounded conditions and daily extremes of temperature, dis­solved-oxygen concentration, pH, and specific conductance at station 2 were out of phase by approximately 12 hours with station 3. Once the dam was removed, the pattern at station 3 shifted and converged with the pattern at station 2. The offset before removal may be related to a lag time resulting from a decrease in velocity through the impoundment.

Total nitrogen and suspended-sediment concentrations increased upon the initial dam breach but were within the range of concentrations measured from March 2001 through April 2002 over varying flow conditions at station 1. Total nitrogen concentration at station 4 was 4.66 milligrams per liter upon the initial breach of the dam, an increase of approximately 0.5 mil­ligram per liter compared to concentrations measured within 24 hours before removal (range before removal was 3.92 to 4.19 milligrams per liter). Suspended-sediment concentrations at station 4 ranged from 22 to 98 milligrams per liter during dam removal compared to a range of 2.8–3.0 milligrams per liter in the preceding 24 hours. Mobilization of reduced forms of nitro­gen, mainly NH4+, did not cause an appreciable decrease in dis­solved-oxygen concentration in the reach downstream of the dam. Correlation between flow and sediment data indicates that the flow events re-occurring from less than 1 to 1.5 years (1,100-5900 cubic feet per second) are likely to have sus­pended-sediment concentrations similar to the maximum con­centration measured during dam removal.

Dam removal did not appreciably alter the downstream benthic-invertebrate community in the vicinity of stations 4 or 5. In contrast, there was a short-term shift at station 3 from pre­dominantly Gammarus amphipods and Tanytarsus midges before removal to more Ephemeroptera, Plecoptera, and Tri­choptera (EPT) taxa 2 weeks after removal. This shift may be the result of different sampling methods used before and after removal. Alternatively, Caenis and other EPT taxa may have exploited newly established riffle habitat created by dam removal. Approximately 1 year after dam removal, the inverte­brate community at station 2 was comparable to the other free-flowing reaches; dominance switched from Caenis to Gam­marus and the midge composition was characterized by more nutrient intolerant species.

The changes observed in the fish assemblage are not beyond the scope of natural fluctuations related to climate and water levels. Because each station was sampled only once before and after dam removal and because fish assemblages can vary naturally, this study was unable to discern any changes in the fish assemblage related to the removal of Good Hope Mill Dam.

Table of Contents

     Purpose and Scope
     Description of Study Area
Study Design and Methods
Effects of Removing Good Hope Mill Dam on Characteristics of Conodoguinet Creek
     Response of the Channel Bed and Banks
     Water-Quality Variations
          Continuously Measured Water-Quality Characteristics
          Nutrients and Suspended Sediment
     Response of the Benthic-Invertebrate Community
     Response of the Fish Assemblage
     Aquatic Habitat
Limitations of the Study
Summary and Conclusions
References Cited     

This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.

View the full report in PDF 4,251 KB

For more information about USGS activities in Pennsylvania contact:
USGS Pennsylvania Water Science Center
215 Limekiln Road
New Cumberland, Pennsylvania 17070
Telephone: (717) 730-6960
Fax: (717) 730-6997
or access the USGS Water Resources of Pennsylvania home page at:

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