Birdlife of Houston, Galveston, and the Upper Texas Coast

Geology, Climate, and Habitats

geology, climate, and habitats The Upper Texas Coast exists as a discrete entity only in a geopolitical sense, extending from the Sabine River at the Texas-Louisiana border south to the western edge of Brazoria County. As we define it, the region comprises seven counties: Jefferson, Chambers, Galveston, Harris, Waller, Brazoria, and Fort Bend. The area and its major centers of development are depicted on map 1. Together these counties encompass 6,405 square miles of land. The total area of the UTC (land and water combined) is 7,635 square miles, roughly equivalent to the size of Massachusetts. The region is virtually flat, with little relief between the coast and the most inland counties (for example, Houston lies between approximately 50 and 60 feet above sea level). Only in northwestern Waller County does the landscape slope into rolling hills; and even at its highest point, the UTC ascends to only 249 feet. geology Perhaps the most noteworthy aspect of the geology of the Upper Texas Coast is how little of it is visible. The ecoregions of the UTC consist of four major habitats that exist at or near sea level (map 2). The surface of the UTC is largely coastal plain. It is inclined toward the Gulf of Mexico at about 5 feet/mile, and with few exceptions exhibits no relief. There are no sea cliffs, offshore stacks, mountains, canyons, or tall riverbanks, and their absence has a profound effect on the types of birds that do and do not occur here. At and below the surface, most of the UTC is composed of muddy sediments derived from inland mountain ranges that were worn down by the combined forces of wind and water. Over millions of years, rivers, many of which originate far inland, carried this sediment toward the Gulf of Mexico. Approaching the sea, the water’s momentum decreased , and its ability to carry suspended loads decreased . Ultimately, the forces of gravity exceeded that of river flow, and silts and sands ended their long journey by settling to the bottom. These sediments built broad deltas that compacted and sank downward, only to be covered and recovered with additional layers of fine particles. In this manner, inch by inch, year by year, the land between Houston and Galveston Island—indeed most of the UTC—was formed. The rich soils that blanket this coastal plain support various agricultural products, but their finegrained texture, which inhibits drainage, makes them especially suitable for the cultivation of two regionally notable crops: rice and crawfish. In turn, the thousands of seasonally flooded acres devoted to these products, as well as the water that covers them, have become important resources for the region’s waterbirds, contributing to the number and diversity of large waders, shorebirds, and waterfowl. These concentrations of birds attract birders, photographers , and hunters who contribute to the region’s economy in many ways. Geologists estimate the sediments along the margin of the UTC to be as much as 50,000 feet thick. The incalculable weight of this sediment layer has impacted even deeper materials. One effect of the coast’s own weight is that thick, deeply buried layers of salt, deposited during the drying of an ancient Triassic sea that existed perhaps 200 million years ago, have been compressed and heated (Spearing 1991). In certain places these salt layers were squeezed toward the surface as tall columnar formations known as salt domes. Infrequently, these salt domes proceed upward until they are close enough to the land’s surface to raise a dimple. The town of High Island (an island of elevated land surrounded by marsh), complete with stores, homes, roads, a school, and several Houston Audubon Society bird sanctuaries, sits atop Table 1. Area and Elevation of the UTC Counties Minimum Maximum Elevation Elevation County Area (mi2) (ft) (ft) Brazoria 1407 0 146 Chambers 616 2 73 Fort Bend 876 46 127 Galveston 399 0 23 Harris 1734 6 171 Jefferson 937 0 42 Waller 514 110 249 geology, climate, and habitats 5 such an uplift, although it barely projects from the surrounding coastal marsh and prairie. Such elevations may seem paltry, but their value to plants, wildlife, and humans is significant. Each additional inch of elevation allows the establishment of more species of plants that cannot tolerate the salty coastal marshes. For example, eastern baccharis , an abundant coastal shrub, can grow on soil only eight inches above the brackish marsh surrounding it. At an elevation of just 14...