Photo Analysis of Marsh Changes Associated with Delta Sedimentation and Faulting near the Mouth of the Colorado River, Texas Gulf Coast

By

WHITE, WILLIAM A., and TREMBLAY, THOMAS A.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,

CALNAN, THOMAS R.

Texas General Land Office, Austin, TX

   Extensive salt-water marshes are located on the Colorado River Delta and Matagorda Peninsula along the central Gulf Coast of Texas. Comparison of marsh distribution interpreted and digitized on historical and recent aerial photographs shows that since the 1950’s, along the west side of the delta in Matagorda Bay, marshes have remained stable or have expanded as a result of delta progradation. In contrast, delta marshes in East Matagorda Bay have eroded, and on Matagorda Peninsula just east of the delta, marshes have been lost because of submergence along an active surface fault. The most extensive marsh expansion since the 1950’s occurred in the southwest corner of the delta on a subdelta at the mouth of an artificial cut. From 1956 through 1979, the subdelta prograded about 800 m, and 90 ha of marsh was established. The subdelta continued prograding until the 1990’s, extending the edge of the marsh another 500 m into the bay and increasing the area of marsh by an additional 80 ha. On the east side of the Colorado River Delta, marsh erosion occurred at rates averaging approximately 1.5 m/yr from 1956 to 1995, accounting for a marsh loss of about 50 ha. The most extensive loss of marshes since the 1950’s, however, occurred along an active growth fault that intersects Matagorda Peninsula east of the delta. Approximately 200 ha of interior salt marsh was lost because of submergence on the downthrown side of the fault. Over time, the fault trace has become more visible on aerial photographs.