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The State of the Bay Galveston Bay Area Project

Freshwater Wetlands

The distribution of fringing and freshwater wetlands in the Lower Galveston Bay watershed occurs along a generally south to north salinity gradient. Freshwater wetlands are found inland and at points farther to the north along bayous and near the mouths of rivers.

Fresh Marsh

Fresh marshes are primarily found in areas where rainfall runoff accumulates (old meander scars of rivers and streams found in Harris, Fort Bend and Brazoria Counties), where rivers and streams provide a water source (oxbows and marshes associated with the fluvial morphology of existing rivers), and locations where fresh groundwater is exposed in a surface depression (interdunal swales on Galveston Island and Bolivar Peninsula).

The fresh water in freshwater marshes is sufficient to maintain a low salinity suitable for such plants as sedges, rushes and coastal arrowhead (Sagittaria lancifolia). In low, wet areas, the exotic and invasive water hyacinth (Eichhornia crassipes) can be found, while panic grasses (Rhynchospora spp.) and spiny aster (Chloracantha spinosa) can be found in higher areas.

Many of the freshwater wetlands found in the Lower Galveston Bay watershed exist in complexes on the coastal prairie. The term “pothole” is used to describe these small, well-defined, freshwater wetland depressions. Prairie pothole complexes consist of potholes and one-to-two-feet-tall pimple mounds (sometimes called mima mounds). The hydrology of prairie pothole complexes can be very diverse with deeper potholes being saturated for up to six months at a time. Neighboring pimple mounds may be semi-arid for most of the year (Moulton and Jacob 2000).

Freshwater wetlands also exist on the Chenier Plain in the eastern portion of the Lower Galveston Bay watershed. Marsh habitats in this area are vulnerable to inundation by salt water during storms. Tropical storm Francis (1998) and Hurricane Ike (2008) both had storm surges that pushed saline water into freshwater marshes on the eastern side of Galveston Bay leading to the death of much of the standing biomass. It may take decades for these marshes to recover; this will happen only with sufficient precipitation to lower the salinity in the water and sediment.

Forested Wetlands

Forested wetlands are found on the floodplains of rivers and streams that cross the Texas coastal plain. In the Galveston Bay system, this community is located almost exclusively in the Trinity River valley. Forested wetlands may occur as bottomland hardwood forests or swamps (Moulton and Jacob 2000). The dominant plant species in the swamp community is bald cypress. The plant community also includes buttonbush (Cephalanthus occidentalis), water elm (Planera aquatica), and water hickory (Carya aquatic) (White and Paine 1992).


Freshwater Wetlands Distribution Trends

Historic Trends

In the Lower Galveston Bay watershed, the majority of wetland losses during the last 50 years can be attributed to the loss of freshwater wetlands (White et al. 1993; Jacob and Lopez 2005; Lester and Gonzalez 2008). White et al. (1993) estimated that of the 35,120 acres of emergent wetlands lost during the 1950 to 1989 time period, 73 percent (25,640 acres) were freshwater wetlands. This equates to a loss of nearly 641 acres per year. White et al. (2004 ) also found that freshwater wetlands decreased by 1,082 acres on Galveston and Follets Island and Bolivar Peninsula between the 1950s and 2002.

The acreage of forested wetlands increased by 177 percent (3,610 acres) between the 1953 and 1989. Almost all of this gain was due to 1) succession, the natural conversion of emergent and scrub/shrub habitats to forest, and 2) the invasion of Chinese tallow, an exotic species of tree with a high tolerance of saturated soil, rapid growth potential and low wildlife value.

Recent Trends

An analysis of the NOAA C-CAP land cover data for the five counties surrounding Galveston Bay (Brazoria, Chambers, Galveston, Harris, and Liberty), showed net losses of freshwater wetlands which totaled 25,787 acres, representing a loss of 1,826 acres per year. Of that amount 15,823 acres of freshwater wetlands were lost to development. The other losses were due to the conversion of freshwater wetlands to non-wetland classifications. Some losses were due to changes in hydrology, which converted the wetland to upland vegetation suitable for grazing.

Work by Jacob and Lopez (2005) estimated that the Lower Galveston Bay watershed lost approximately three percent of its freshwater wetlands to development between 1992 and 2002 (9,052 acres of freshwater emergent, forested, and scrub/shrub classes). Most of the loss occurred in Harris County, which lost at least 13 percent of its freshwater emergent wetlands in the same period, with over half of that loss occurring between 2000 and 2002. The spatial distribution of these losses
can be seen in the map to the right.

 

The estimates document a continued and substantial loss of wetlands in the Lower Galveston Bay watershed. Jacob and Lopez (2005) estimates equate to a 0.3 percent per year loss of classified freshwater wetlands in Lower Galveston Bay watershed from 1992 to 2002. The NOAA C-CAP study (1996-2005) estimates an annual rate of loss of 2,599 acres of freshwater wetlands or 0.3 percent.

Table 1.  Acreage of estuarine and freshwater wetland in the five counties of the Lower Galveston Bay watershed from 1996 to 2005. Data Source: (NOAA 2006; Lester and Gonzalez 2008).
 
Acreage
 
Wetland Classification 1996 2005 Total Change 1996 to 2005 Annual Change 1996 to 2005 Percent Change 1996 to 2005
Estuarine Emergent 163,029 163,228 +199 +20 0%
Freshwater Emergent 169,746 168,068 -1,678 -168 -1%
Freshwater Forested 564,715 546,451 -18,264 -1,826 -3%
Freshwater Scrub/Shrub
75,061 69,016 -6,045 -605 -3%

Total

972,551 946,764 -25,787 -2,579 -3%

 

Freshwater Wetland Preservation

Loss of freshwater wetlands continues and is of great concern. The loss is primarily associated with conversion to uplands for suburban and urban development and agricultural purposes. The storm surge associated with Hurricane Ike inundated freshwater and intermediate marshes north of East Bay. It will take time for the salinities of these wetlands to reach normal levels. Marsh restoration and creation projects have added a significant amount of freshwater, brackish and salt marshes, and similar efforts may be required to re-establish the freshwater marshes destroyed by Hurricane Ike. However, habitat preservation projects are needed to slow the loss of freshwater wetlands in the Lower Galveston Bay watershed.

 

Literature Cited

Jacob, J. S. and R. Lopez. 2005. Freshwater, Non-tidal Wetland Loss Lower Galveston Bay Watershed 1992-2002: A Rapid Assessment Method Using GIS and Aerial Photography. Webster, TX: 73 pp.

Lester, L. J. and L. A. Gonzalez. 2008. Galveston Bay Status and Trends Final Report. Houston, TX, Texas Commission on Environmental Quality, Galveston Bay Estuary Program: 83 pp.

Moulton, D. W. and J. S. Jacob. 2000. Texas coastal wetlands guidebook. Bryan, Texas, Texas Sea Grant College Program: 66 pp.

NOAA. 2006. "Gulf Coast Land Cover." Web Page of the National Oceanic and Atmospheric Administration (NOAA), Coastal Services Center, Coastal Change Analysis Program (C-CAP).

White, W. A. and J. G. Paine. 1992. Wetland plant communities, Galveston Bay System. Webster, Texas, Galveston Bay National Estuary Program Publication GBNEP-16.

White, W. A., T. A. Tremblay, R. L. Waldinger and T. R. Calnan. 2004 Status and trends of wetland and aquatic habitats on barrier islands, upper Texas coast, Galveston and Christmas Bays. Final report. , Texas General Land Office and NOAA under GLO Contract No. 03-057.

White, W. A., T. A. Tremblay, E. G. Wermund and L. R. Handley. 1993. Trends and status of wetland and aquatic habitats in the Galveston Bay system, Texas. Webster, Texas, Galveston Bay National Estuary Program Publication GBNEP-31.

 

 

Related Pages:

Estuarine Wetlands
Habitat

 

Wetlands Land Cover Map
Galveston Bay Wetlands Land Cover Map, Houston Advanced Research Center
Map of wetland habitats in the Lower Galveston Bay watershed. Data source Coastal Change Analysis Program land use/land cover data (NOAA 2006).

 

 

PHOTO OF A FRESHWATER MARSH IN GRIMES PRAIRIE AT ARMAND BAYOU NATURE CENTER. IMAGE COURTESY ANDREW SIPOCZ.
Freshwater marsh in Grimes Prairie at Armand Bayou Nature Center.  Image courtesy Andrew Sipocz.

 

 

Image of forested wetland of the lower Galveston Bay watershed.  Copyright istockphoto.com/Aaron Frankel.
Forested wetland of the Lower Galveston Bay watershed. Copyright istockphoto.com/Aaron Frankel.

 

 

2002 Freshwater Wetlands
Map of wetland distribution from 1992-2002, from Jacobs and Lopez, 2005.
Freshwater wetlands in the Lower Galveston Bay watershed in 2002.  Areas in red were wetlands in 1992, but had been converted to developed land.  Areas in green were classified as wetlands in 1992 and 2002.  (Jacob and Lopez 2005).

 

 

NOAA Gulf Coast Land Cover Data
Thumbnail image of NOAA gulf coast land cover data
NOAA Coastal Services Center

 

 

2008 Hurrican Ike Storm Surge
INFRARED SATELLITE VIEW OF VEGETATION NORTH AND EAST OF EAST BAY; (LEFT) BEFORE HURRICANE IKE AND (RIGHT) AFTER HURRICANE IKE (IMAGE TAKEN SEPTEMBER 28, 2009). LIVING VEGETATION IS DISPLAYED IN RED AND INUNDATED AREAS ARE IN BLUE-GREEN. IMAGE COURTESY OF NASA/GSFC/METI/ ERSDAC/JAROS, AND U.S./JAPAN ASTER SCIENCE TEAM.
Infrared satellite view of vegetation north and east of East Bay; (left) before Hurricane Ike and (right) after Hurricane Ike. Living vegetation is displayed in red and inundated areas are in blue-green. Image courtesy of NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan Aster Science Team.

 
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