In April, most of the data for the index were provided by LCRA staff. Because LCRA staff no longer measures transparency in streams, transparency data for river sites were contributed by the Colorado River Watch Network, a group of volunteers who help monitor the waterways of the lower Colorado River watershed in Texas. Seven of 15 lower Colorado River basin monitoring sites earned excellent scores. Dissolved oxygen concentrations and water temperatures were optimal throughout the basin. Two sites, San Saba River and Lake Marble Falls, received poor scores that were attributable to rainfall runoff increasing the bacteria concentrations. The other Highland Lakes sites and upper basin tributaries earned excellent scores.
Results were mixed for the Colorado River downstream of Austin. The Columbus site received an excellent score. Downstream of Austin, however, nitrate concentrations were elevated and caused two sites – Bastrop and Smithville – to receive only good marks. The nutrient concentrations at these two sites were elevated but less than what was measured in March because flows were greater in April. La Grange earned a fair rating. The two sites nearest the bay, Wharton and Bay City, received fair ratings.
Bacteria are the most important determinant of the water quality rating. Sites with bacteria concentrations greater than 399 Most Probable Number/100 milliliters are rated poor. This was the case with the San Saba River and Lake Marble Falls sites. Four Colorado River locations with elevated bacteria concentrations but less than 399 MPN/100 ml fell into the fair category.
Seven sites earned excellent scores:
- Lakes Buchanan, Inks, LBJ and Travis
- Llano and Pedernales rivers
- Colorado River at Columbus
Two sites earned a good score:
- Colorado River at Bastrop and at Smithville
Four sites earned a fair rating:
- Colorado River at Austin, La Grange, Wharton and Bay City
Two sites received poor marks:
- San Saba River near San Saba
- Lake Marble Falls
Highland Lakes region
The Highland Lakes and major upstream tributaries, the Llano and Pedernales rivers, earned excellent ratings with optimal water quality measurements supportive of healthy aquatic life. The San Saba River and Lake Marble Falls received poor ratings because of elevated bacteria concentrations. The bacteria were likely washed in with rainfall runoff. All monitoring sites in the region reported ideal oxygen levels, and low bacteria and nutrients.
Lakes Buchanan and Travis had transparency to depths of 4.7 and 13.7 feet, respectively. Lake Travis had the greatest transparency of the monitored sites in April. Inks Lake transparency was measured at almost 7 feet. Lakes LBJ and Marble Falls also were transparent to a depth of 4.7 and 4.2 feet, respectively. Transparency in the San Saba River was about 2 1/2 feet. The Pedernales River’s was approximately 3 feet. The Llano River was more transparent and was measured at 4 feet.
Water temperatures in the upper basin ranged from a low of 59 F (15 C) in the San Saba River to 68 F (20 C) in the Llano River. Elsewhere in this geographic area, water temperatures were primarily in the mid 60s.
Downstream water quality improved somewhat in April. The nutrient, or nitrate, concentrations at Bastrop and Smithville were still somewhat elevated but less than in March. Further downstream, the nutrient concentrations were higher in April than March. Aquatic plants continue to be very abundant in the river and seem to help filter suspended solids in the water. Bacteria concentrations were somewhat elevated. Healthy dissolved oxygen concentrations supportive of fish communities were measured throughout the lower river. The Colorado River at Columbus earned an excellent score.
Water temperatures in the lower basin ranged from a low of 68 F (20 C) at Austin to 74 F (23.6 C) at Columbus. Water temperatures were typically in the lower 70s at other downstream sites. With a transparency tube reading of 1/2 foot, transparency was lowest at Bay City. In the Colorado River downstream of Austin, the greatest transparency was measured at Wharton with 4 feet. The transparencies downstream of Austin averaged slightly less than 3 feet.