Lake Conroe is a man-made lake, constructed in 1973 along the West Fork of the San Jacinto River. It is a water supply reservoir, meaning that water levels are maintained as close to the permitted normal pool level as possible to assure the largest supply of water in times of drought. What it also means, however, is that there is little or no storage space for storm water.
The lake’s normal pool elevation is 201 feet above mean sea level (msl) for water supply. An automated network of real-time rainfall and stream flow monitoring stations continuously observes the rainfall and lake level. The State permit requires that any waters above elevation 201 must be “passed through” the reservoir by releasing it through one or more of the five gates. If the water is not passed from the bottom of the gates, rising water will overtop the gates and flow downstream anyway. Continued rises in the water level could overtop the dam and risk failure of the structure. The excess waters released through the gates flow down the West Fork of the San Jacinto River. The gates are situated at about the center of the dam, which sits at the southern end of the lake.
It’s important to realize that the San Jacinto River Authority only releases flood water when the lake level exceeds 201 feet. And, even when the gates are releasing water, the dam still holds back a significant amount more than what’s being passed through the gates.
For example, if the gates are open 5 feet (and they are 200 feet wide), water is being released through an area 5 feet by 200 feet, equaling 1,000 square feet. But, since the dam is 11,000 feet long, the corresponding height on the rest of the dam is being held back and calculates to be 5-foot deep and 10,800-foot wide or 54,000 square feet.
Releasing water before the lake reaches the 201-foot elevation would cause the streambed to fill prematurely. Then, even a very small amount of rain could cause the river to flow out of its banks unnecessarily. The gates, in reality, regulate how much of the water from the 444-square-mile watershed above Lake Conroe enters the streambed and can in many cases minimize the effects of a severe storm. Additionally, pre-releases would need to occur for approximately two weeks to lower the lake level significantly, which is well beyond accurate weather forecasting.
Did you know?
The length of the dam is approximately 11,000 feet. Each gate is 40 feet wide, so the gates cover an area of the dam 200 feet long. When the lake reaches the 201-foot elevation, the gates are raised to keep water from spilling over them. Water is then released from beneath the gates.