Effects of hardened low-water crossings on stream habitat, water quality, and periphyton in four streams at the Fort Polk Military Reservation, Vernon Parish, Louisiana, October 1998 through November 1999

At the request of the U.S. Army Joint Readiness Training Center and Fort Polk, the U.S. Geological Survey is studying the potential effects of hardened low-water crossings on selected streams located in the eastern section of the Fort Polk Military Reservation, Louisiana. The crossings are constructed on Big Brushy Creek, an unnamed tributary to Big Brushy Creek, an unnamed tributary to East Fork of Sixmile Creek, and Little Brushy Creek. The hardened low-water crossings were constructed to allow military vehicles to pass through riparian zones and streams with minimal effect on existing vegetation and aquatic life. These modified crossings could affect stream habitat, water quality, and periphyton upstream and downstream from the crossings. Low-water crossing sites were evaluated from October 1998 through November 1999 for potential effects on stream habitat, water quality, and periphyton. Habitat data collected upstream and downstream from each of the four crossing sites indicated banks were stable, bank angles were steep, and canopy angles were low before and after crossing modifications. Habitat data collected at each of the low-water crossing sites indicated banks were not stable, bank angles were low, and canopy angles were high before crossing modifications. The low-water crossing modifications increased bank and slope stability near the crossing, which can reduce soil erosion into the streams. Water-quality data collected before and after modifications indicated streams were acidic, with field measurements of pH ranging from 4.4 to 6.5, and were poorly buffered, with field measurements of acid neutralizing capacity ranging from 1.6 to 6.7 mg/L (milligrams per liter). Field measurements of specific conductance ranged from 16 to 36 microsiemens per centimeter at 25 degrees Celsius. Concentrations of nutrients and major inorganic ions were low (less than 3 mg/L). Relative to concentrations of other major inorganic ions in the streams, silica concentrations were high, ranging from 7.7 to 13.0 mg/L. Concentrations of all water-quality constituents analyzed were typical of small streams in this area. Few differences in stream habitat and water quality were observed before and after crossing modifications at Big Brushy Creek, unnamed tributary to Big Brushy Creek, and unnamed tributary to East Fork of Sixmile Creek. Geomorphic channel units at the hardened low-water crossings were similar to those observed at the upstream and downstream reaches. The height and grade of the low-water crossings allowed movement of sand, silt, and gravel in the crossings, similar to bed sediment movement observed in the upstream and downstream reaches. Statistical results indicated that concentrations of 15 of the 17 waterquality constituents analyzed were not significantly different upstream and downstream from these three crossing sites. Although concentrations of two water-quality constituents were statistically different, the waters containing these constituents were not chemically different. Changes in stream habitat and differences in water quality were observed after crossing modification at Little Brushy Creek. The height above the streambed and grade of the hardened low-water crossing restricted flow, and caused: (1) an increase in velocity across the hardened crossing, (2) an increase in depth Louisiana State University, School of Renewable Natural Resources (about 1.5 feet) at the upstream reach, and (3) dissolution of the carbonate foundation material. These changes probably caused the chemical differences observed in the concentrations of calcium, magnesium, bicarbonate, and acid neutralizing capacity between the upstream and downstream sites. Based on results of habitat assessment and water-quality analyses, the U.S. Army reconstructed the low-water crossing at Little Brushy Creek by lowering the crossing height to eliminate the ponding effect upstream from the crossing and by replacing the carbonate foundation material with calcium-free aggregate to eliminate the chemical differences observed downstream from the crossing. Periphyton were used to monitor effects of low-water crossing modification because of their sensitivity to changes in stream water quality and habitat. Statistical analysis of 325 periphyton samples indicated that the periphyton in the streams were not affected by the low-water crossing modifications. The variability observed in the analysis of the periphyton samples was due primarily to seasonal changes in periphyton community structure. Major composition of the periphyton community included diatoms, green algae, and a red algal species. Analysis of periphyton community samples collected at sites near the lowwater crossings indicates that these streams are oligotrophic (low nutrient concentrations and low biological production) and generally unaffected by anthropogenic activities.