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5: Water Census

Yvonne C. Allen1 and Glenn C. Constant2

1 U. S. Geological Survey, National Wetlands Research Center, Coastal Restoration Field Station, Parker Coliseum, Baton Rouge, Louisiana
2 U. S. Fish and Wildlife Service, Baton Rouge Field Office, Parker Coliseum, Baton Rouge, Louisiana

Within the Atchafalaya Basin Floodway System (ABFS), there is a complex structure of lakes, rivers, canals, and spoil banks formed by both natural and engineered processes. The distribution and quality of water within each water management unit (WMU) is primarily driven by water level and condition found in the main river channels flowing through the ABFS. Diverse physical morphologies within each WMU, however can result in very different patterns of water distribution among the WMUs. River level gages have been established at many well-traveled locations throughout the basin but very little synoptic information has been available at more remote locations.

The amount, quality, duration, and flow rate of water in large part define the availability and quality of habitats for flora and fauna living in the basin. It is therefore critical to gain a better understanding of the dynamics of water distribution patterns at basin-wide and WMU scales. We classified 33 Landsat TM 5 and 7 (TM) images from 1983-2008 into categories of land, open turbid water, open swamp water, shallow turbid water and shallow swamp water. Each image was captured during leaf-off conditions to optimize delineation of ground conditions. Turbid water distribution was compared to concurrent ground truth sampling to validate the classification. This series of classified imagery can be used to evaluate the distribution of land, water and turbid water through time and also to predict the impact of various flooding scenarios. A historical record of turbid water distribution could also allow managers to identify open water areas that have consistently received high levels of sediment. These open water areas may be at risk for conversion to land due to sediment accretion.

Contact Information: Yvonne C. Allen, U. S. Geological Survey, National Wetlands Research Center, Coastal Restoration Field Station, Parker Coliseum, Baton Rouge, LA 70803; phone: 225 578 7478

Richard H. Day1, Kenneth W. Krauss1, Thomas W. Doyle1, S. Andrew2

1 U.S. Geological Survey, National Wetlands Research Center, Lafayette, Louisiana
2 IAP World Services, National Wetlands Research Center, Lafayette, Louisiana

Healthy tidal freshwater forests are usually located at or above the level of man high water. In coastal Louisiana, many areas of baldcypress (Taxodium distichum) forest exist below the level of mean high water due to the combined effects of global sea level rise and local subsidence. Wind plays a greater role than astronomical tides in controlling water levels in the Barataria Basin, where tidal swamps are isolated by levees from any major riverine influence. Freshwater input is mainly from rainfall. A water level recorder was installed within a tide-influenced baldcypress forest in the Barataria Basin in August 2004. Salinity of the groundwater was measured in shallow wells every 4-5 weeks. A hydrograph of the water level data shows a tidal signature but it is dominated by wind driven water levels as depths varied from 90 cm above to 15 cm below the forest floor. The average daily wind speed and direction, measured at the Louis Armstrong New Orleans International Airport approximately 35 km from the study site, coincide with dominant peaks and valleys of the hydrograph. Groundwater salinity within the baldcypress forest ranged from 0.6 to 1.6 ppt until the Hurricane Rita storm surge in September 2005, after which groundwater salinity rose to 4.5 ppt. Prolonged drought after Hurricane Rita resulted in elevated groundwater salinity throughout 2006. These data will be used in ongoing studies of forest growth and productivity. Baldcypress at this site tolerate chronic exposure to saltwater, but are in danger of decline as relative sea level continues to rise.

Contact Information: Richard H. Day, U.S. Geological Survey, National Wetlands Research Center, 700 Cajundome Blvd., Lafayette, Louisiana 70506; phone: 337-266-8557; fax: 337-266-8586; email: dayr@usgs.gov

Richard H. Day and Thomas W. Doyle

U.S. Geological Survey, National Wetlands Research Center, Lafayette, Louisiana

The Atchafalaya River basin, which contains the largest contiguous riverine forested wetland in the United States (3584 km2), receives 30% of the combined flows of the Red and Mississippi Rivers. The entire Atchafalaya basin is bordered by high levees to protect the surrounding landscape from flooding. The basin is also divided into subbasins bounded by lower natural and artificial levees. Water flow through the basin tends to be channelized. Hydrological exchange between subbasins and major channels occurs during high water overbank flooding, but this exchange is usually restricted to small channels which reverse flow regularly to fill and drain the subbasins with the passing of flood peaks. This limited exchange results in stagnation and hypoxic conditions, and the forested wetlands within the subbasins tend to be waterlogged for long periods of time. Hydrology within the forests is also influenced by rainfall, evapotranspiration, and subsurface drainage.
We examined long term records of water level gages in the Atchafalaya River and other in-channel gages and compared them to data collected from continuous water level recorders we installed within the forested wetlands to measure flooding above the surface of the forest floor. We compared hydroperiods within the forests to the hydrograph of the nearest source of riverine input to gain an understanding of the range of conditions within the forested environments in relation to the river connectivity of each site. The seasonality, depth, and duration of flooding within the forested wetlands control the composition of forest species and the flow of nutrients for sustained productivity. Riverine input also delivers sediment, which by its deposition causes gain in elevation, lowering flooding frequency and forcing change in forest composition.

Contact Information: Richard H. Day, U.S. Geological Survey, National Wetlands Research Center, 700 Cajundome Blvd., Lafayette, Louisiana 70506; phone: 337-266-8557; fax: 337-266-8586; email: dayr@usgs.gov

Christopher M. Swarzenski, U.S. Geological Survey, Louisiana Water Science Center, Baton Rouge Louisiana

Since the early 1900’s, an extensive network of levees built for flood control has prevented the direct inflow of Mississippi River water into most deltaic wetlands in south Louisiana. The supply of freshwater and sediment needed by these wetlands to flourish and keep pace with sea-level rise has been reduced and, in many places, completely eliminated. In its place, within the last 50 years, the GIWW (Gulf Intracoastal Waterway) has become the largest distributary of Mississippi River water to coastal Louisiana wetlands. The GIWW is a major east-west trending ship channel traversing the entire Louisiana coast. Following natural hydraulic gradients, the GIWW captures water and sediment from the southward flowing Lower Atchafalaya River and Wax Lake Outlet, and distributes this river water to wetlands up to 30 to 50 miles east and west of the intersections. Most of the water in the Atchafalaya River originates from the Mississippi River. The passive GIWW flow is controlled by seasonally changing differences in water surface elevations between the Atchafalaya River and adjacent watersheds and becomes predictable when stage of Lower Atchafalaya River at Morgan City is above 3 ft NAVD88. The GIWW has become the largest and frequently only source of Mississippi River water to many parts of coastal Louisiana. The ship channel functions as the hydrologic and ecological equivalent of a freshwater diversion. The reach of the flow in the GIWW is much greater than most constructed diversions.

Contact Information: Chistopher M. Swarzenski, U.S. Geological Survey, Louisiana Water Science Center, Baton Rouge, 3535 S. Sherwood Forest Blvd., Ste 120, Baton Rouge, Louisiana 70816; phone: 225 298 5481; email: cswarzen@usgs.gov

Mark C. Kasmarek and Natalie A. Houston

U.S. Geological Survey, Texas Water Science Center–Gulf Coast Programs Office, Woodlands, Texas

The Chicot aquifer (in Holocene- and Pleistocene-age sediments), Evangeline aquifer (in Pliocene- and Miocene-age sediments), and Jasper aquifer (in Miocene- and Oligocene-age sediments) are the three primary aquifers in the Gulf Coast aquifer system in Texas. The hydrogeologic units are laterally discontinuous, fluvial-deltaic lenticular deposits of gravel, sand, silt, and clay that dip and thicken from northwest to southeast. The units thus crop out in bands inland from and approximately parallel to the coast, becoming progressively more deeply buried and confined toward the coast. The Chicot aquifer outcrop, which comprises the youngest sediments, is the closest of the aquifer outcrops to the coast, followed farther inland by the Evangeline aquifer outcrop and then farthest inland by the Jasper aquifer outcrop.

The U.S.G.S., in cooperation with the Harris-Galveston Subsidence District, the City of Houston, the Fort Bend Subsidence District, and the Lone Star Groundwater Conservation District, publishes an annual report on the water-level altitudes and water-level changes for the Chicot, Evangeline, and Jasper aquifers and compaction of subsurface sediments in the Chicot and Evangeline aquifers in the Houston-Galveston region. The 2008 report (Scientific Investigations Map 3031) shows potentiometric surfaces as low as 200 feet below sea level for the Chicot aquifer in central Harris County; 300 feet below sea level for the Evangeline aquifer in northern Harris County, and 150 feet below sea level for the Jasper aquifer in southern Montgomery County. Additionally, for the period 1977–2008, the report shows water-level rises of as much as 200 and 260 feet in the Chicot and Evangeline aquifers, respectively, in southeast Harris County.

The Houston-Galveston region is the largest urban area in the U.S. affected by land-surface subsidence caused by ground-water withdrawals. Sustained withdrawals cause water levels in the aquifers to decline, which in turn causes depressurization and dewatering of the clay lenses. Subsequently, the individual grains of the clay lenses begin to realign and compress. Measured subsidence data using spirit-leveling and GPS techniques indicate that as much as 10 feet of subsidence has occurred in areas of southeastern Harris County. For the same area, data derived from subtraction of a 1915–17 DEM from a 2001 DEM show that land-surface elevation has declined as much as 13 feet. Land-surface subsidence is especially problematic for coastal areas having low topographic relief. Impervious land-surface cover, surficial clay in the Chicot aquifer, and Gulf of Mexico low-pressure systems with storm surge and high rainfall, combine to make areas affected by subsidence more flood prone.

Contact Information: Mark C. Kasmarek, U.S.G.S. Texas Water Science Center–Gulf Coast Programs Office, The Woodlands, Texas, 19241 David Memorial Dr. Suite 180, Conroe, TX 77385; phone:936-271-5318; fax:936-271-5399; e-mail: mckasmar@usgs.gov

Cliff R. Hupp1, Charles R. Demas2, Daniel E. Kroes2, Richard H. Day3, and Thomas W. Doyle3

1 U.S. Geological Survey, 430 National Center, Reston, Virginia
2 U.S. Geological Survey, 3535 Sherwood Forest Blvd., Baton Rouge, Louisiana
3 U.S. Geological Survey, 700 Cajundome Blvd., Lafayette, Louisiana

Sediment deposition and storage are important functions of forested bottomlands, yet documentation and interpretation of sedimentation processes in these systems remain incomplete. Our study was located in the central Atchafalaya Basin, Louisiana, a distributary of the Mississippi River and contains the largest contiguously forested riparian wetland in North America, which suffers from high sedimentation in some areas and hypoxia in others. We established 20 floodplain transects reflecting the distribution of depositional environments within the central Basin and monitored general and local sediment deposition patterns over a three-year period (2000–2003). Deposition rate, sediment texture, bulk density, and loss on ignition (LOI, percent organic material) were determined near or just above artificial markers (clay pads) located at each station per transect. Transect mean sedimentation rates ranged from about 2 to 42 mm/yr, mean percent organic material ranged from about 7% to 28%, mean percent sand (.63 m) ranged from about 5% to 44%, and bulk density varied from about 0.4 to 1.3. The sites were categorized into five statistically different clusters based on sedimentation rate; most of these could be characterized by a suite of parameters that included hydroperiod, source(s) of sediment-laden water, hydraulic connectivity, flow stagnation, and local geomorphic setting along transect (levee versus backswamp), which lead to distinct spatial sedimentation patterns. Sites with low elevation (long hydroperiod), high hydraulic connectivity to multiple sources of sediment-laden water, and hydraulic damming (flow stagnation) featured the highest amounts of sediment trapping; the converse in any of these factors typically diminished sediment trapping. Based on aerial extent of clusters, the study area potentially traps 6,720,000 Mg of sediment annually, of which, 820,000 Mg represent organic materials. Thus, the Atchafalaya Basin plays a substantial role in lowland sediment (and associated contaminant) storage, including the sequestration of carbon. Findings on local sedimentation patterns may aid in management of flow to control sediment deposition and reduce hypoxia.

Contact Information: Cliff Hupp, US Geological Survey, 12201 Sunrise Valley Drive, MS 915B, Reston, VA 20192; phone: 703 648 5207; email: crhupp@usgs.gov