Rio Grande Delta Marshes

Area Description

The Rio Grande and the Arroyo Colorado are the main fluvial elements of the delta. The Rio Grande forms the international boundary between Mexico and the United States and it discharges directly into the Gulf of Mexico. The Arroyo Colorado is a distributary of the Rio Grande and it empties into the Laguna Madre. Unlike streams of the upper and central Texas coast, the Rio Grande and Arroyo Colorado do not have extensive swamps and freshwater marshes associated with them. However, prior to the construction of dams, floodways, and levees along its course, the Rio Grande typically overflowed its banks annually, depositing new sediment and sending water into a variety of discharge meander channels (including the Arroyo Colorado) in the delta. These flood waters constituted significant freshwater input into the marshes of the Rio Grande Delta, but in the past 50 years dams and flood control projects have virtually eliminated this source of freshwater. The marshes are now dependent on rainfall alone for freshwater input. In addition, much of the native vegetation, topography, and drainage patterns of the Rio Grande Delta have been destroyed or greatly altered by man for agricultural, municipal, and industrial uses.

The cessation of annual flooding may have produced changes in the marsh vegetation of the delta, but the nature and magnitude of change (if it has occurred) is unknown because the ecology of the marshes is poorly known. Indeed, in this semi-arid region it is difficult to distinguish the boundaries between: 1) salt water marshes and brackish water marshes, 2) brackish water marshes and fresh water marshes, and 3) brackish water marshes and the drier vegetated saline flats. Major reasons for these difficulties are the flat topography (average slope is 0.46 to 0.61 m per 1.6 km), a climate where evaporation exceeds precipitation, the proximity of the Laguna Madre and the prevailing southeasterly winds that carry salt spray inland, and extremely high tides during hurricanes that push salt water inland along drainage courses. Marshes have been distinguished based on elevation, vegetation, and soil and surface moisture.

A correlation on the distribution pattern of marsh communities along an elevation gradient has been reported in literature. At low elevations, a community of Batis maritima, Salicornia virginica, and Sueda linearis grades almost imperceptibly into slightly higher elevation characterized by Borrichia frutescens, Batis maritima, and Monanthochloe littoralis, which in turn grades upward into a community of Spartina spartinae. Recently, it was reported that the vegetation of a typical brackish water marsh is organized into three zones along an elevation gradient. At the lowest elevations there is a distinct zone dominated by maritime saltwort, (Batis maritima). At the lowest elevations in this zone where rainwater remains the longest, stands of California bulrush, (Scirpus californicus), occur. An intermediate elevation zone supports shoregrass, (Monanthochloe littoralis), as the dominant species. A third (highest elevation) zone is dominated by Gulf cordgrass, (Spartina spartinae). The upper margin of this zones grades gradually into a shrub-grassland community that occurs on clay dunes (lomas) in the area. Each of the zones are distinguished by a distinctive signature in multispectral videography. It was also found that salt water marshes had the same dominant species (except Scirpus californicus), but that the position of the M. littoralis and Spartina spartinae zones were reversed in the salt water marsh, i.e. Spartina spartinae occurred at intermediate elevation and Monanthochloe littoralis occurred at higher elevation. Thus, the positions of these two communities may be used in distinguishing between brackish water marshes and salt water marshes.