The tide along the Texas coast is predominantly diurnal, meaning that there is one 

 high and one low in a (lunar) day. The difference between mean high water and mean 

 low water (mlw) is called the mean range of tide, because it contains both diurnal and 

 semidiurnal components. The difference between mean higher high water and mllw is 

 called the diurnal range of tide and encompasses only the single highest high and lowest 

 low in a day. The mean tide range in the Gulf of Mexico is 1 .4 ft at the NOAA National 

 Ocean Service (NOS) tide gauge located at the Galveston Pleasure Pier on the Gulf of 

 Mexico side of Galveston Island. Secondary NOS gauges that have been placed at Port 

 O'Conner, located just inside Pass Cavallo, and at Lavaca Bay, located at the north end of 

 the bay, indicate mean and diurnal tide ranges that are nearly the same, about 0.8 ft. 

 Thus, as the tidal wave propagates into and across the bay, bottom friction damps the 

 small semidiurnal component almost completely. 



Extreme changes in water level in Matagorda Bay that define navigation-controlling 

 depth are primarily vidnd induced and accompany weather fronts that occur from about 

 October to May. Seasonal variability in water level is also substantial as compared to the 

 mean range of tide in Texas bays and estuaries. Because extreme low waters along the 

 inland coastal waters of Texas are meteorologically controlled and not deterministic 

 (related to the astronomical tide), the Galveston District has defined a local navigation 

 datum called mean low tide (mlt). This topic has been discussed by Kraus et al. (1997). 

 The mlt datum in Matagorda Bay hes 1 ft below the National Geodetic Vertical Datum 

 (NGVD) of 1929, as shown in Table 1 in Kraus et al. (1997). The geodetic datum 

 NGVD 29 should not be confused with the tidal damm of mean sea level (msl), because 

 they are not necessarily equivalent. Because NGVD monuments are lacking in the area 

 and the NOS is presently updating tidal datums for the new 19-year National Tidal 

 Datum Epoch 1980-1998, for the purpose of numerical modeling within the DMS, in this 

 study mlt will be assumed to he 1 ft below msl. 



Freshwater inflow to Matagorda Bay is moderate (Harwood 1973; Ward, Wiersema, 

 and Armstrong 1982; Mueller and Mathews 1987) and consists primarily of the water 

 discharges of the Colorado River Diversion Channel and the Lavaca River, discussed 

 further in Chapter 3. Matagorda Bay and East Matagorda Bay were once a continuous 

 water body into which the Colorado River discharged. Since the time of the early 

 European settlers, the Colorado River had been plugged by a log jam that entrapped 

 sediment and caused flooding in neighboring areas. In 1929, the logjam was freed by 

 combined dredging and dynamiting. The discharged sediments rapidly crossed 

 Matagorda Bay to form a delta that reached Matagorda Peninsula in 1935, creating a new 

 bay now called East Matagorda Bay (Bouma and Bryant 1969). 



In 1992, the Colorado River was rerouted to Matagorda Bay by the Galveston 

 District to supply fresh water to the system. East Matagorda Bay is no longer connected 

 to Matagorda Bay because of the lock system on the Gulf Intracoastal Waterway 

 (GIWW) that facihtates navigation past the Colorado River. The circulation and changes 

 in water level in East Matagorda Bay have been measured and numerically modeled by 

 Kraus and Mihtello (1996, 1999). 



Pass Cavallo 



Matagorda Bay was originally connected to the Gulf of Mexico through a single 

 permanent inlet, Pass Cavallo, located on the southwest end of the bay (Figure 2). Price 

 (1952) found that the large estuaries of Texas tend to possess inlets in their southwest 

 comers and freshwater inflows in their northeast comers. Stabihty of the southwest 



Chapter 1 Background and Problem Statement 



