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The Impacts of the Low Water Sills on the Lower Pearl River Basin

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The Impacts of the Low Water Sills on the Lower Pearl River Basin The Impacts of the Low Water Sills on the Lower Pearl River Basin The Nature Conservancy Lower Pearl Partnership June 30, 2005 C. S. Haase, Ph.D. Hydrogeologist, Lower Mississippi River Program The Nature Conservancy Southern Region Amy Smith Kyle and Debbie Bird Current and former Coordinator, Lower Pearl Partnership The Nature Conservancy of Louisiana/Mississippi 2 Introduction This report explores the impacts of low-head overflow dams (locally known as low water sills, which is the nomenclature that will be used in this report) on the ecological integrity of the lower Pearl River Basin. Sills and flow diversion structures can potentially impact ecologically important aspects of the hydrologic regime, geomorphology, sediment regime, and water chemistry of river systems (Schumm, 2005). The report will give an initial review of the technical literature to outline potential impacts of sills to the lower Pearl River Basin, and provide a summary and analysis of site-specific studies that can be used to identify and evaluate which of the potential impacts have occurred and to what extent. The history of the flow control structures on the lower Pearl River, the suspected impacts of the low water sills on the river’s hydrology and flow regime, fluvial geomorphology, water quality (sediment regime and water chemistry) and riverine wildlife is highlighted. Much of the discussion focuses on the Pool’s Bluff Sill because it is on the mainstream of the Pearl River and the majority of the available data are relevant to it. The lower Pearl River Basin (which includes the East and West Pearl rivers and associated distributary channels and bayous, the Pearl River Navigation Channel, and the lowermost Bogue Chitto) are influenced by three sills and a flow diversion structure. Just south of Bogalusa, the Pearl River divides at Pool’s Bluff Sill into two major branches, the Pearl River Navigation Canal (West Pearl River), and the Pearl River (East Pearl River). The Navigation Canal and the West Pearl River converge just below Lock No. 1 and flow southward as the West Pearl River. The lower Pearl River Basin extends from near Bogalusa, Louisiana, to the mouth of the West and (East) Pearl Rivers. These rivers empty into the Rigolets and Lake Borgne. During the Nature Conservancy’s Pearl River Conservation Area Planning meetings, the team of experts from local groups, academia, state and federal agencies from Louisiana and Mississippi identified low water sills as one of the seven highest ranked threats to the Pearl River system. Low water sills are considered to be a major threat to resident riverine aquatic fauna and migratory fishes. Background Information There are several different flow control structures on the lower Pearl River that change or divert water flow (Figure 1.). Low water sills are three to four-foot high barriers that stretch across the river. Sills are used to create backwater pools above the locks in order for enough water to accumulate to fill the lock’s rectangular concrete chambers. Once the water in the lock is equal to the level of water in the sill pool, the gates can be opened and allow boats to travel through. Sills also maintain minimum water levels in the canal. There are three sills in the lower Pearl River basin (Table 1.). The sills were constructed by the U.S. Army Corps of Engineers (USACE) in the lower Pearl River as part of the West Pearl River Navigation Project. This Navigation project was authorized by the 3 1935 Rivers and Harbors Act and began in 1938. Construction was halted during WWII and then completed in 1956 (G. Walker, pers. commun. 2004). The Navigation project extends from the mouth of Bogalusa Creek in Bogalusa, Louisiana for 58 miles south to the mouth of the Pearl River (Kohl 2003). The channel was constructed to be seven feet deep with a bottom width of 100 feet in the river portion and 80 feet wide in the canal section. The three low water sills in the lower Pearl include Pool’s Bluff Sill, Bogue Chitto Sill (Figure 2.) and an Unnamed Sill. Pool’s Bluff Sill is located south of Bogalusa on the mainstem Pearl River and above Lock No. 3. The Pool’s Bluff Sill is 350 feet wide and 11.83 feet high. The Bogue Chitto Sill is 250 feet wide and is located on the Navigation Canal where the Bogue Chitto River crosses the canal and north of Lock No. 2. The Unnamed Sill, known locally as the “Talisheek Sill,” is located immediately upstream of Lock No. 1, close to the confluence of the canal and the West Pearl River. All sills are made of reinforced concrete. The Pool’s Bluff and Bogue Chitto Sills were constructed to control water levels in the navigation channel. The three locks on the Navigation Canal were also completed as part of the West Pearl Navigation Project (Figure 3.). The three locks in the 20.2-mile section have inside chamber dimensions of 65 x 310 feet. The locks were used to maintain river and canal water levels to allow navigation through the canal. Another lock is located at the entrance of the canal at the NASA Test Facility. NASA has an existing 20-mile long and 12-feet deep Navigation Channel on the East Pearl River that extends from Lake Borgne to the NASA Test Facility in Hancock County. This lock is used to transport equipment to the facility. The use of the West Pearl Navigation Canal decreased substantially in the 1970s and was no longer considered a commercial waterway. The project was nominated for deauthorization in 1974. USACE suspended maintenance dredging of the Navigation Canal and placed the lock structures in “caretaker status” in 1975. However, at the request of local interests in Slidell and Bogalusa, Louisiana, the economic and environmental feasibility of maintaining the project was reevaluated. The results of the feasibility study indicated that maintenance of the waterway was economically justified. Maintenance funds to continue dredging and reopen the existing waterway were provided in 1988. Dredging began in 1989, but was discontinued because the inflated heelsplitter (a freshwater mussel designated as threatened under the Endangered Species Act) was believed to be found in the area. In 1991, it was determined that an Environmental Impact Statement (EIS) would have to be prepared before a decision could be made to reopen the waterway. An EIS was completed in 1994 and identified 23 species of concern that may exist in the area. In 1995 the project was again considered in “caretaker status.” Only one person is assigned to the canal to make sure the locks are functional for recreational users and the area is maintained. USACE informed The Nature Conservancy that the project is in the initial stages of the deauthorization process and hope to get funding in FY 2006 to further the process (G. Walker, pers. commun. 2004). As part of the deauthorization process the 4 USACE will assess the current condition of the West Pearl Navigation Canal, research alternatives to stabilize the project, and explore who may be interested in taking over the project and land, such as the State of Louisiana (G. Walker pers. commun. 2004). There is also a weir, flow diversion structure, at Walkiah Bluff on the East Pearl River. In the past, during low flow periods the majority of the flow in the East Pearl River in Mississippi had been diverted to the West Pearl River in Louisiana due to natural channel alterations at Wilson Slough. The alterations left the Pearl River channel (~18 miles to the mouth of the canal) near Picayune and Holmes Bayou (a Louisiana-designated Natural and Scenic Stream) almost dry during low-flow conditions, as Wilson Slough continued to grow. This situation was addressed in 1997 and 1998 through a cooperative effort between Mississippi and Louisiana and local entities, which called for construction of a weir and closures to restore flows into the river channel. The project was completed in 1998, but is still being monitored by USACE (Mississippi Department of Environmental Quality 2000). Hydrologic and Flow Regime Impacts Potential major hydrologic and geomorphic features associated with sills are illustrated in Figure 4. Hydrologic impacts of these features (backwater pool formation) will be discussed in this section and their geomorphic impacts (plunge pool development, depositional bar formation, and channel alteration) will be discussed in the following section. Figure 4. A generalized cross section parallel to flow direction of a sill (low-head overflow dam) illustrating upstream and downstream hydrologic and geomorphologic impacts (based on Figure 3 from Juracek 1999). The principal hydrologic effect of sills is the creation of a backwater pool or reservoir immediately upstream of the sill (Juracek 1999, Schumm 2005). The dimensions and extent of such a pool are strongly dependent on pre-dam channel geometry and the dimensions of the sill. For most sills, depths of the backwater pool immediately behind the structure rarely exceed 10 to 15 ft, and the backwaters typically extend several hundred to a few thousand feet upstream from the dam. On relatively large water courses such as the Pearl River the storage capacities of reservoirs formed by sills are typically small compared to the daily flow volume of the river (Dunne and Leopold 1978). For example, the post-Ross Barnett Dam median daily flow at Bogalusa LA is 4,709 cubic 5 feet per second (cfs) which is equivalent to a water volume of ~ 4.07 x 108 cubic feet delivered, on average, daily to the backwater pool of Pool’s Bluff Sill. A very generous estimate of the reservoir volume of the backwater pool at Pool’s Bluff is ~1.0x107 cubic feet, which is ~ 40 times less than the volume of water delivered daily, and suggests that the residence time for water in the backwater pool must be very short (on the order of hours at most).
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