| In March, scientists began adding small amounts of red dye to provide a more precise determination of how much water is in the Colorado River downstream of Austin. |
 mall amounts of nontoxic red dye are rolling down the Texas Colorado River downstream of Austin this spring as scientists under contract to the Lower Colorado River Authority help measure how fast the river's waters travel downstream.
The dye-tracer study is intended to gather data about the volume of water traveling in the river for use in a computerized model of the Colorado River downstream of Austin. This model will help determine the impact of the proposed LCRA-SAWS Water Project, which is designed to conserve and develop water supplies to help meet the future needs of the lower Colorado River basin and the San Antonio area.
The project, a model of cooperative regional water planning, is expected to benefit cities, farmers, businesses and recreation in the two regions while also meeting environmental inflow needs. The six year feasibility study began in 2004 to determine if the project will benefit both regions without harming the Colorado River or Matagorda Bay.
Dye study being conducted twice
The dye study is being conducted twice: this past March and again later this spring or early summer.
The scientists added small amounts of Rhodamine dye to the river at several points downstream of Austin in early March. Because of heavy winter rain, the testers delayed the study by a month.
"It would have been preferable to get a low flow of around 700 cubic feet per second (cfs)," said Jim Patek, who conducted similar studies when he worked for LCRA eight years ago and is now a consultant for engineering firm Parsons Water Infrastructure, which is working with LCRA on the study. "Those happen in the winter, but winter is also the rainy season."
In March the river flow was between 8,000 to 9,000 cfs, Patek said. The next test is planned for April or May. During those months flow is expected to be 3,500 cfs, a medium flow, when water is released downstream for rice irrigation.
The dye appears fluorescent bright orange under ultraviolet wavelengths and can be visible in the river for as long as 24 hours. During March testing, the dye was visible in an area of about 35 feet for less than one-quarter mile.
Study could help determine amount of flows in the Colorado
The dye-tracer study will provide scientists with a more precise determination of how much water is in the Colorado River downstream of Austin, That information is important, as a major component of the LCRA-SAWS Water Project calls for developing one or more "off channel reservoirs" near the Colorado River to store water that would otherwise flow unused into the Gulf of Mexico. (Water would be pumped from the river to the reservoirs.)
That captured water would help meet the future needs of San Antonio; it would also help supply the needs of agricultural users in the Colorado River's coastal region. These users currently depend mainly on releases from the Highland Lakes upstream of Austin. One of the critical factors that will determine the project's feasibility is whether the river holds enough water not only to meet the needs of San Antonio and the Colorado's agribusiness consumers but also to meet the environmental needs of the river channel and its estuary, which depends on a balance of fresh and salt water to maintain the health of marine life.
The dye study can help provide those answers.
"We need to set up a model of the lower river so we can play with the 'what if' scenarios," Patek said. "It's necessary to know the water velocity, and the depth and width of the river at different flow rates. LCRA has some of this data, but in addition, we need fairly detailed information in some places."
Scientists use sophisticated tools to measure flow
The scientists used a global positioning satellite unit as they move downstream, measuring the water's speed in terms of its flow rate ("flow" means the volume of water, in cubic feet per second), as well as the river's width (using a laser range finder) and its depth (using a fathometer, or depth sounder).
"When you know the width, depth and speed, you can calculate the velocity of the water moving through the area," Patek said.
That's when the scientists released about a quart of dye into the water, tracking the "peak" (or amount of concentration of dye in the water) with a fluorometer and following it as it flowed down the river.
The time it took for the dye to travel helped measure velocity. "If we catch the peak a mile downstream and it's gone in 15 minutes, the water is moving at four miles an hour," Patek said.
The study team is following guidelines from the U.S. Environmental Protection Agency and the U.S. Geological Survey limiting the amount of dye that can be released in the river, Guidelines limit the amount of dye that can be released in the course of a week's study to a maximum of five gallons.
All releases are occurring downstream of Town Lake in Austin. The March portion of the study tested the river's flow at three segments, from Bastrop to Smithville, Webberville to Utley and Columbus to Wharton.
Mary Morales is a writer specializing in water issues at LCRA. Write her at info@lcra.org.
Related links:
LCRA-SAWS Water Project Web site
U.S. Geological Survey: Technical memorandum on dye tests |
