Nutrient and Sediment Monitoring in Inflows to Texas Bays and Estuaries

Science Center Objects

The USGS Texas Water Science Center is evaluating the variability of nutrient and sediment concentrations and loads entering Texas bays and estuaries across a range of hydrologic conditions in Galveston Bay (inflow from the Trinity and San Jacinto Rivers), Matagordo Bay (inflow from the Colorado River), San Antonio Bay (inflow from the Guadalupe River), and Nueces Bay (inflow from Nueces River).

Bays and estuaries depend on the delivery of freshwater, nutrients, and sediment to maintain physical conditions that support a diversity of estuarine life.   The amount of nutrients and sediment delivered to an estuary affects the water quality, productivity, and characteristic of a coastal ecosystem. Whereas nutrients are required to sustain life, excess nutrients and suspended sediment can alter the nutrient cycle balance and can be detrimental to the health of organisms living in and using coastal waters.

With the demand for water in Texas anticipated to increase with population growth, sustaining adequate freshwater inflows into bays estuaries while meeting water supply needs may become an increasing challenge for resource managers. Assessing the processes driving the delivery of freshwater, nutrients, and sediment into Texas estuaries is essential for improving our understanding of freshwater inflow needs.

As part of this study,  USGS Oklahoma-Texas Water Science Center scientists are:

  • Collecting data to characterize and quantify freshwater volume, nutrients, and sediments entering various bays and estuaries in Texas 
    • Galveston Bay (inflows from Trinity River and San Jacinto River)
    • Matagordo Bay (inflows from Colorado River)
    • San Antonio Bay (inflows from Guadalupe River)
    • Nueces Bay (inflows from Nueces River)
  • Determining potential sources of nutrients in freshwater inflows
  • Evaluating the potential for estimating sediment concentrations on a continuous basis using surrogate parameters. 

Nutrients

Nitrogen and phosphorus compounds occur naturally in coastal streams and rivers but also are commonly applied to land as commercial fertilizers and livestock waste. Nitrogen is present in water as nitrite and nitrate anions, as ammonium cations (all inorganic nitrogen), and as part of organic solutes.

Phosphorus is not as abundant as nitrogen and often is the limiting element for plant growth. Usually phosphorus is present as phosphate in natural waters, and much of the phosphorus in streams attaches to particulate matter and is unavailable for uptake by plants. Orthophosphate species are the predominant dissolved phosphorus forms in most streams.

Nutrients that are not utilized by crops or stored in the soil can runoff to streams in overland flow or infiltrate with groundwater recharge. Poor water quality caused by an abundance of these nutrients in an estuary can stimulate the excessive growth of phytoplankton, reduce dissolved oxygen (DO) levels, and potentially lead to fish kills.

Colorado River near Matagorda, TX

High sediment load in the Colorado River near Matagorda, TX during a high flow event.

Suspended Sediment

Similarly, high sediment loads delivered to an estuary can alter water quality. Concentrations of suspended sediment are affected by natural conditions (e.g., soil erosion and streambed re-suspension) and can be affected by human activities (e.g., construction, timber harvesting, certain agricultural practices, and hydraulic alteration). An increased sediment load delivered to an estuary can reduce water clarity and light penetration in the water column.

Suspended sediment also plays a major role in the transport and fate of nutrients and other contaminants. In Texas, periods of high flow in streams and rivers flowing into a coastal ecosystem are usually caused by local rainfall or releases from upstream reservoirs made in response to rainfall further 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.

Texas Bays

Recent work in these river systems suggests that the response of nutrient and sediment concentration to hydrologic conditions varies among the river basins and according to the cause of increased flows (e.g., reservoir releases versus local precipitation events). However, despite the event-based or basin-specific variations in the response of nutrient and sediment loading to the bays, the data thus far supports the idea that large pulses of nutrients and sediments are transported to the bays during peak inflow periods.

Therefore, the timing and supply of nutrients has the potential to significantly affect bay health and fisheries resources. During initial study efforts on these river systems, observations supported the potential to use the backscatter signal recorded by acoustic Doppler velocity meters (ADVM) and in situ field measurements to estimate suspended sediment concentration as well as potentially estimate certain nutrient concentrations. 

Data Collection

USGS Streamgage on Trinity River surrounded by floodwaters

USGS Streamgage on the Trinity River near Wallisville, Texas, during flood conditions

Streamflow Data Collection

Streamflow data are computed at USGS stations in the lower reaches of each basin using the index velocity method. In the index velocity method, continuous records of stage and velocity are used to compute discharge records from concurrent measurements of stage, velocity, and discharge. To develop and maintain index velocity ratings USGS Oklahoma-Texas Water Science Center scientists continuously measure stage and velocity and obtain instantaneous discharge measurements over a range of hydrologic conditions.

Water Quality Data

Water quality samples are collected at each streamgaging station seasonally and over a range of hydrologic conditions. Because most of the sediment and nutrient load delivered to bays and estuaries occurs during periods of high flow, most water quality samples are collected during storm events. 

Surrogate Evaluation

Acoustic Doppler Velocity Meters (ADVMs) are primarily used to measure water velocity when applying the index velocity method.  ADVMs also output a return pulse strength indicator, called backscatter. Although backscatter is most often used to assure the quality of velocity data, it also can serve as an indicator of the concentration of sediment in the meter’s measurement volume. Scientists collect sediment samples from the river while the ADVM is deployed and relate the sediment concentrations to backscatter measurements. USGS Oklahoma-Texas Water Science Center scientists use data from sediment samples collected at each streamgage to develop regression models between the sediment concentrations and corrected backscatter data. If a model can be developed upon collection of additional data, suspended sediment concentration can be predicted in real-time using ADVM backscatter data. The USGS has developed surrogate models for suspended sediment concentrations in the Trinity River and Guadalupe River (include link to sites?) and is currently collecting additional data to develop surrogate models for the San Jacinto River and Colorado River.