Why did the Corps let Canyon Lake get so high?
By Wade Reinhardt and Judy Scott
Canyon Lake has many benefits such as boating, swimming, and fishing; however, one of the main purposes of the lake is flood control and in the month of November it once again was needed for that job. Since the normal conservation pool level of the lake is 909 ft above mean sea level (msl) and the uncontrolled spillway's elevation is 943 ft msl, Canyon Lake is able to store up to 34 feet of flood water in the lake to protect downstream areas.
During October and through mid-November, upstream rains raised the lake level to 921ft msl and higher than normal releases for that extra twelve feet were occurring up until November 20. The rains were not over yet and on Sunday, November 21 heavy rains caused downstream areas to begin flooding. Therefore, Corps officials made a decision to lower the release gates at Canyon Dam on Sunday November 21 even though the reservoir still held a few feet of floodwater. The reason for that was to lower the amount of water flowing into the river. That way the dam releases would not be adding to high river levels expected downstream from the heavy precipitation and run-off. More heavy rains fell on Monday November 22 and dropped large amounts of precipitation onto many portions of the Guadalupe River's watershed, especially downstream from the dam. So much rain fell that flooding occurred in downstream areas as close as New Braunfels, where the Guadalupe River overtopped the Common Street Bridge. This was occurring from runoff and precipitation alone since the dam's release gates were nearly closed. Conditions were even worse downstream from New Braunfels because the flooded flows of Comal, Blanco and San Marcos Rivers all eventually joined into the flooded Guadalupe River. People in communities in the lower Guadalupe basin such as Seguin, Gonzalez, Cuero and Victoria saw even more serious flooding problems. If the dam had still been releasing a large volume of water, the flooding conditions downstream would have been a lot worse. Also, the inflow into the lake had peaked at 10,100 cfs on November 22; therefore, the dam reduced the downstream flooding by holding those flows in the reservoir.
Back at Canyon Lake, closing the release gates meant our lake began to rise. Many people were concerned that another spillway overflow might occur as it did in July of 2002. It took 30-50 inches of rain for that event to happen. Even though we had significant rains upstream during this flood event, the lake rose only to 927.59 ft msl giving it over 15 feet of flood storage remaining. On December 1st the river levels downstream were low enough that the Corps was able to open the dam release gates to the maximum which made releases nearly 5000 cubic feet per second.
Unfortunately high lake levels caused the Corps of Engineers to have to close their parks and boat ramps since most facilities are underwater. Work has been done over the years to design park facilities that can withstand high water and wind, but park cleanup and minor repairs are always expected after this type of event. After the lake returns to its normal level, debris removal and cleanup will be an on going effort throughout the winter to prepare for the spring season.