ABSTRACT

Extensive salt-water marshes are located on the Colorado River Delta and Matagorda Peninsula along the central Gulf Coast of Texas (Fig. 1). 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 fault that intersects the surface.

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, Tiger Island Channel. 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 (Fig. 2). In 1991, as part of the Colorado River Diversion Project (Wilber and Bass, 1998), a dam was constructed across Tiger Island Channel, causing progradation of this subdelta to cease.

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. For the period 1957 to 1972, McGowen and Brewton (1975) estimated erosion rates of approximately 2 m/yr on the east side of the delta. 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 (Fig. 2). 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. On the fault's downthrown side, which is toward the Gulf, marshes have been replaced by open water and barren intertidal flats (Table 1), indicating a rate of subsidence and relative sea-level rise that exceed marsh vertical accretion on the downthrown side. Marsh loss is not as great on the upthrown side. The fault apparently extends to the northeast into East Matagorda Bay and affects island marshes to the southeast of the fault beyond the visible extent of the fault trace as shown on Figure 2. The length of the visible part of the fault is about 5 km. The average length of 40 faults that cross wetlands mapped along the upper Texas coast (White and Morton, 1997) is about 3.8 km, with the longest being a little over 13 km. Of the 40 faults mapped, about 25% were 5 km or more in length, indicating that the fault crossing Matagorda Peninsula is among the longest. Additional faults have been mapped on Matagorda Peninsula to the northeast. One is downthrown toward a nearby oil and gas field, suggesting a possible association with oil and gas production as reported for some faults on the upper Texas coast by White and Morton (1997).

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Figure 1. Index map showing location of Colorado River Delta and Matagorda Peninsula.

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Figure 2. Map of Colorado River Delta illustrating progradational phases of Tiger Island subdelta and marsh development from 1956 to 1992 and an active fault on Matagorda Peninsula along which marshes have been lost to submergence on the downthrown side.

Table 1. Loss of salt marsh associated with the active fault crossing Matagorda Peninsula. In the vicinity of the fault, marsh areas mapped on 1950's aerial photographs were lost and replaced by 1992 with the following habitats.

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