The water quality in estuaries and bays and the health of these coastal ecosystems are affected by sediment and nutrient loads transported by streams. Large sediment loads delivered to an estuary or bay can degrade water quality. Concentrations of suspended sediment are affected by natural conditions (such as soil erosion and streambed resuspension) and can also be affected by human activities (such as development, timber harvesting, certain agricultural practices, and hydraulic alteration). Nutrients are needed to sustain life, but excess nutrient loads from human activities may cause unbalanced and unhealthy changes in water quality that are harmful to aquatic organisms. Nitrogen and phosphorus are two known nutrients of concern. Poor water quality caused by an abundance of these nutrients can stimulate the excessive growth of phytoplankton, promote algal blooms, reduce dissolved oxygen levels, and cause fish kills. Approximately 60 percent of coastal rivers and bays in the United States have been moderately to severely degraded by excess nutrients. Water quality in rivers constantly changes in response to rainfall in the watershed, and increased sediment and nutrient loads in rivers often occur during periods of high flow.

In Texas, periods of high flow in rivers flowing into coastal ecosystems are usually caused by local rainfall or by releases from upstream reservoirs made in response to rainfall farther upstream in the basin. The increase in rain and resultant flooding can increase sediment erosion and nutrient runoff into coastal rivers and consequently increase sediment and nutrient input into estuaries and bays. In 2009, the U.S. Geological Survey, in cooperation with the Texas Water Development Board, began evaluating the variability of sediment and nutrient loads in the lower reaches of the Trinity River during a variety of hydrologic conditions. Discharge, sediment concentration and sand/fine break, and nutrient concentration data (including nitrate and phosphate concentrations) were collected at USGS streamflow-gaging station 08067252 Trinity River at Wallisville, Texas, to gain a better understanding of the hydrologic and water-quality characteristics for the Galveston Bay coastal ecosystem. This ongoing study is designed to help characterize the sediment and nutrient load transported into Galveston Bay as related to localized periods of high flow and releases of water from reservoirs upstream in the watershed.

First posted April 29, 2011