May 2011 FINAL ASSESSMENT OF ‘LESSONS LEARNED’ FROM THE OPERATTIONS OF EXISTING FRESHWATER DIVERSIONS IN SOUTH LOUISIANA Prepared for Coastal Protection and Resttoration Authority of Louisiana 450 Laurel Street, Suite 1200 Chase Tower North Baton Rouge, Louisiana Prepared by Baton Rouge, Louisiana FINAL ASSESSMENT OF ‘LESSONS LEARNED’ FROM THE OPERATIONS OF EXISTING FRESHWATER DIVERSIONS IN SOUTH LOUISIANA LDNR Contract No. 2503-10-81 Task Order No. 10 GEC Project No. 0027.8500210.000 Prepared by 9357 Interline Avenue Baton Rouge, Louisiana 70809 Phone – 225/612-3000 COASTAL PROTECTION AND May 27, 2011 RESTORATION AUTHORITY OF LOUISIANA Wax Lake Delta - Cover photograph from LSU Coastal Louisiana Ecosystem Assessment 2010 Landsat 5 Thematic Mapper Satellite Image, 17 Nov. 05. Image provided by John Barras, U.S. Geological Survey EXECUTIVE SUMMARY EXECUTIVE SUMMARY The primary purpose of river diversions is to supply freshwater, nutrients, and sediments to restore and maintain coastal wetlands that have been lost over the last 70 years. Ecosytem restoration plans, especially those developed after Hurricane Katrina, recommend more river diversions. Existing river diversions have been designed for different purposes. Types of diversions include large-scale freshwater diversions, crevasses, siphons, and flood control spillways. The large- scale diversions (Caernarvon, Davis Pond, and Bayou Lamoque) were designed and constructed to divert freshwater and associated nutrients, not necessarily large amounts of sediments. The recently constructed West Bay, Delta Wide, and Channel Armor Gap crevasse projects were designed to divert freshwater, sediments, and nutrients. The Wax Lake Delta was created by the creation of Wax Lake Outlet as a flood control structure. Delta National Wildlife Refuge was created by the creation of Cubit’s Gap. Several small siphon diversions are also currently in operation or have been operated in the past (including White’s Ditch, Violet, La Reussite, West Point a la Hache, and Naomi) to divert freshwater and nutrients. The Bonnet Carré and Morganza spillways are opened occasionally to divert excess water from the Mississippi River for flood control purposes. The Old River Control Structures are operated continuously to maintain the distribution of flow between the Mississippi River and Atchafalaya Rivers. Ecosystem changes, such as river diversions, will have both benefits and drawbacks. The challenge is to understand and quantify these effects so that effective management decisions can be made; and to enhance the benefits and mitigate the drawbacks. The effects of large river diversions on estuarine and marine organisms are a concern, as well as the effects of high nutrient loads coupled with low sediment input on marsh vegetation. The objectives of this report are to examine the available data, literature, and reports on existing river diversions in south Louisiana to develop a “Lessons Learned” document. Specifically, this document summarizes the effects of diversions on coastal elements including: soils, vegetation, wildlife, and fisheries. Although abundant information on Mississippi River diversions is available, only information pertaining to effects of existing freshwater diversion projects on these particular coastal elements was analyzed for this report. The initial step in the development of this report was to collect and organize the existing literature on river diversions in coastal Louisiana. The literature was a mix of traditional, peer- reviewed literature (white literature), as well as agency reports, conferences, and meeting proceedings (grey literature). As a result, a literature database was developed to provide a central location for these documents. To data, approximately 1,272 documents have been recorded in this database. The diversions that appeared to build the most land were those that diverted significant amount of sediment and included the Atchafalaya Delta, Cubit’s Gap, and the Delta Wide Crevasses Project. The Atchafalaya and Wax Lake deltas have rapidly evolved through the processes of seaward channel extension and bifurcation as well as lobe fusion and upstream growth by coarse ES-i sediments (primarily fine sand). It was estimated that approximately 153 km2 above the -0.6 m contour was created from 1981 to 1995. Maintenance dredging has caused differences in the Atchafalaya and Wax Lake deltas. Since dredging ceased on the Wax Lake Outlet in 1980, the delta has been building naturally. Dredged material placement along the channel banks in the Atchafalaya Delta has formed a proficient channel which carries sediment to the Gulf of Mexico and hampers the delta-building process for the surrounding areas when compared to the Wax Lake Delta. Systematic monitoring has shown that a direct correlation exists between growth of the delta with flood duration and volume. The total land gain for the Delta Wide Crevasses (State Project Number MR-09) project is 499 acres at approximately 23 acres/year per crevasse. Elevation gain is impacted by the crevasse angle of orientation and width. Wider crevasses and crevasses oriented at 60 degree angles from their parent channels gained elevation and created subaerial land at rates faster than narrower crevasses oriented 90 degrees from parent channels. Deeper areas take longer to fill because they require more sediment and subaerial expression of the crevasse splay may be delayed. Abnormally high river stages cause more shoaling than normal in the main channel which results in additional dredging. This additional shoal material, combined with high river flows, increases the volume of sediments introduced into the crevasses areas. Cubit’s Gap, an artificial crevasse created in1862, essentially created 48,000 acres of land that became Delta National Wildlife Refuge in 1935. Land is still being lost in the outfall of the West Pointe a la Hache siphon. The Bonnet Carré Spillway deposits sediment (mostly silt and sand) during each opening. In a major flood, the river can deposit more than 12 million cubic yards of sediment on Spillway lands. During the 1973 opening, an area of silty sand (an average of 19 cm thick) was deposited near the mouth of the Spillway. Vegetation habitat in the Breton Sound Basin changed from 1978 to 2000 due to the operations of the Caernarvon Freshwater Diversion. Freshwater marsh was not observed in 1978; however, 628 acres were documented in 2000. Since 1978, brackish and saline marsh decreased, and intermediate marsh increased by 10,582 acres. For Delta Wide Crevasses, lower plant diversity is observed in the late stages of succession as the more stable competitive species begin to dominate. Primary succession can be observed by combining the elevation and vegetation data; new plant growth follows land creation. In 1999, a non-diverse group of pioneer plants were observed. Post-construction of the crevasses (2002), a more diverse community of competitive species became dominant. Plant succession is dynamic at the Atchafalaya Delta and can be divided into three categories. Some species have increased over time and converged on certain elevational zones. Other species are relatively stable over time with elevational shifts attributable to local erosion or accretion. Some species are present over a wide range initially, eventually disappearing at low elevations. Freshwater releases through the Caernarvon Diversion and Davis Pond have increased the amount of submerged aquatic vegetation coverage. Increased phytoplankton growth has been ES-ii noted after Bonnet Carré Spillway openings. Phytoplankton composition changes after the 2008 Spillway opening were similar to those of the 1997 opening. Blue-green algae dominated the phytoplankton community in 2008, but were less severe than in 1997, when a large blue-green algae bloom resulted in lake-wide recreation health advisories. Ammonium concentrations were highest at sites near Caernarvon during months of high flow; however, concentrations at the far site were less variable. Porewater concentrations of ammonium increased with depth and ammonium concentrations were greater at the upstream and transition sites compared to the downstream site. Concentrations of phosphate (PO4) during late spring and early summer were typically higher towards Caernarvon and at the marine end of the estuary. Total phosphorus was also higher during high discharge periods. Distinct zones of high phosphate concentrations were evident during high discharge periods in the winter. Freshwater from Caernarvon may help to maintain low sulfide concentrations. Increased frequency and volume of riverine discharge could potentially decrease sulfide levels. Sulfide concentration was significantly correlated to belowground biomass. Sulfide concentrations during the growing season appeared to be correlated to lower biomass at the sites least affected by the diversion. Introductions of river water may be successful at decreasing chloride concentrations and increasing habitat space for organic rich, freshwater marshes. Average aboveground biomass production was greater at sites near Caernarvon than sites in the lower basin. In marshes affected by Caernarvon, biomass typically occurs in the upper 30 cm of the soil column, and the greatest amount of belowground biomass production generally occurs at the 0–20 cm depth range. Belowground biomass productions in the upper and lower basins were essentially the same with a slightly greater amount at the upper station nearest freshwater input. Wildlife populations generally