Tag Archive for: seasonal

Seasonal Lowering of Lake Conroe Begins August 1

On August 1, the City of Houston Public Works Department, the San Jacinto River Authority (SJRA) and the Coastal Water Authority (CWA) plan to begin lowering the level of Lake Conroe as part of a joint operations plan to mitigate flooding threats during the peak of hurricane season. The plan calls for lowering Lake Conroe gradually through small releases – about an inch a day – until the lake level reaches 199 msl (mean feet above sea level).

Intent of gradual lowering of Lake Conroe is to avoid another 79,000 cubic-feet-per-second release rate as we experienced during Harvey. Until normal flow is restored to the West Fork, Lake Conroe will be lowered temporarily and seasonally to mitigate flood risk.

Lake Conroe to Be Lowered 2 Feet Through September

This initial lowering will last through the end of September to create additional storage in Lake Conroe which could delay releases and minimize release rates during a storm, thereby providing a buffer against flooding for residents who live downstream of the dam.

The next lowering would occur during the peak of the spring rainy season – from April 1 – May 31, 2019. However, that lowering would only be by a foot – to 200 msl, because the rain threat is usually lower in spring.

Only Until Dredging Restores River’s Flow

The joint operations plan calls for continuing to lower Lake Conroe seasonally in this manner while the U.S. Army Corps of Engineers dredges the West Fork of the San Jacinto to restore flow.

Hurricane Harvey deposited tremendous amounts of silt in the West Fork which  physically changed the river’s ability to safely pass water during storms. Hence, the dredging.

Phase-One Dredging Has Already Started

Phase one will go from River Grove Park to the West Lake Houston Parkway bridge. Great Lakes, the contractor hired by the Corps, has 269 more days to complete the initial phase.

Phase-Two Still Needs Funding

The total project could take longer. Community leaders are now working furiously to arrange funding to dredge the remainder of the west fork, including the mouth bar.

Mobilization and demobilization comprise 25 percent of the total phase one project costs (approximately $18 million out of almost $70 million). Having phase two ready to start before phase one ends could save that money, creating extra value for taxpayers and enabling even more dredging, say for instance, on the East Fork.

Temporary Lake Conroe Lowering Could Last Up to 3 Years

The City, SJRA, and CWA will continue the seasonal lowerings for up to three three years. They will monitor progress of the dredging and annually re-evaluate the need to lower Lake Conroe. If the river’s flow is restored before three years, the temporary lowering would cease.

Lakes will Operate under Two Different Strategies

Lake Conroe is located upstream from Lake Houston. Large pre-releases immediately before a storm run the risk of pushing water into Lake Houston at a rate that could flood residents. Therefore, the SJRA will release at a much more gradual, controlled rate and maintain the lower level until the largest flood threat passes.

Lake Houston, as the lowest lake on the river system, can pre-release more safely. Therefore CWA will pre-release from Lake Houston if the National Weather Service predicts more than 3 inches of rain within the San Jacinto River basin in a 48-hour period. Coastal Water Authority will lower Lake Houston to 41.5 feet from its normal elevation of 42.5 feet.

To track lake levels visit:

  • Lake Conroe – www.sjra.net
  • Lake Houston – www.coastalwaterauthority.org

4 Million People Depend on City for Water

Lake Conroe and Lake Houston comprise two of the largest parts of the City’s drinking water system. More than 4 million Houstonians and residents of the greater Houston region rely on water provided by the City.

Posted by Bob Rehak on July 30, 2018

335 Days since Hurricane Harvey

Pros and Cons of Two Alternative Strategies to Lower Lake Conroe

In its April board meeting, the San Jacinto River Authority voted to seasonally and temporarily lower the level of Lake Conroe to reduce flood risk for downstream residents.

Several learned and respected people suggested an alternative strategy in response to pushback from the Lake Conroe Association. Their proposal goes something like this. “As soon as a named storm enters the Gulf, begin lowering the level of the lake.”

On the surface, it sounds like an appealing and fair compromise. Wait until a real threat appears rather than react pre-emptively to an anticipated threat. Both strategies seemingly attain the same objectives.

However, like all difficult decisions, this one contains hidden layers of complexity. Let me attempt to explain the relative pros and cons of each strategy so that you can understand the tradeoffs.

Temporary, Seasonal Lowering

  • Slow, controlled release of one or two inches per day
  • Rate is certain not to flood downstream residents
  • Extra capacity gained in lake reduces flood risk during storm
  • Avoids liability for SJRA
  • Predictability – Lake Conroe residents know what to expect and can plan around it
  • No coordination issues with Lake Houston, which has a different kind of dam
  • May waste water if no storm appears
  • Will inconvenience some boaters with shallow docks on Lake Conroe for six weeks or possibly more  during hurricane season

Wait and See Before Lowering

  • Less risk of wasting water
  • Does not inconvenience boaters or other Lake Conroe residents without imminent cause
  • Flood threat may not come from the Gulf or be a named storm
  • Difficult to release enough water at a SLOW rate to make a difference during a major storm
  • Tropical storms can blow up near coast or traverse Gulf in a couple days
  • Less reaction time would require faster release rate
  • Faster release rate might flood downstream residents living close to river
  • Lake Houston can only discharge 10,000 cubic feet per second through gates.
  • Lowering Lake Conroe two feet at that rate would require approximately 2.5 days.
  • Could erase excess capacity in downstream watershed, which would most likely fill up first if storm approaches from south
  • Weather forecasts cannot accurately predict how much rain will fall or where it will fall within a watershed. Lake Conroe might get NO rain and Lake Houston might get more than it can handle – on top of a rapid release from the Conroe dam.
  • TCEQ recommended against it


The primary objective of lowering Lake Conroe is to reduce flood risk when it is highest for downstream residents. It would also provide an extra margin of safety for Lake Conroe residents, many of whom flooded during Harvey. From that standpoint,  the temporary seasonal lowering has the highest probability of success. Here’s why.

The temporary, seasonal lowering can be carried out at a rate of one or two inches per day as weather and downstream conditions permit. It’s a sure thing.

The wait-and-see strategy carries more risk (from the flood prevention point of view) because of the unpredictability of tropical storms. Sometimes they blow up near the coastline at the last minute. Storms can also easily cross the Gulf in three days, as Alberto did, or change course at the last minute, as Rita did. (Remember the mass evacuation of Houston that turned out not to be necessary?)

If it typically takes a hurricane three days to cross the Gulf, using the wait-and-see strategy requires reducing the level of Lake Conroe eight inches per day (instead of one or two inches per day) to achieve a two foot reduction. That might flood downstream residents in Montgomery and Harris Counties – especially if the storm approaches from the south and loads up the downstream watershed WHILE Lake Conroe is releasing.

Another problem with the wait-and-see strategy is this. What if the storm is not tropical in nature? What if it approaches from the north or west and still dumps eight to 10 inches of rain on us? Engineers would have even less time to release in such a case. The Tax Day Flood in 2016 dumped more than 16 inches of rain in parts of our area in just 12 hours – from thunderstorms that approached from the west with less than two days’ warning.

Lake Conroe can release water ten times more quickly than Lake Houston.

A two-foot lowering would not do much to protect us from another Harvey; it would provide only a few additional hours to evacuate. However, such a lowering would protect us from smaller storms, such as 10-, 25-  or 50-year events, and those are FAR more likely to occur.

Both strategies have a flaw from a precedent point of view. A letter from the TCEQ to the SJRA dated March 26, 2018, states that, “The general rule in this country is that the operator of a dam may permit floodwaters to pass through a dam in an amount equal to the inflow, but will be liable if any excess amount is discharged.”

Hmmmm. That puts the ball squarely back in the City of Houston’s court. It looks as though the City of Houston will have to rely on its right to draw water from Lake Conroe if it wants to lower the level of the lake during hurricane season.

According to Houston City Council Member Dave Martin, the City has not used its allotment since the drought in 2011/2012. The City’s allotment of 100,000 acre feet. Because the lake is approximate 20,000 acres in size, if the City used its full allotment, it could lower the level of the lake by five feet, far more than the 2-foot reduction the LCA is fighting.

All parties should keep in mind that neither strategy is permanent. Lake level reductions would only happen UNTIL other long-term mitigation measures become effective. Those include dredging the river and the installation of additional flood gates on Lake Houston. Dredging is designed to restore the river’s carrying capacity and velocity. Additional gates on Lake Houston would eliminate a downstream bottleneck by equalizing the discharge rates of the two dams on the river.

Posted DATE, 2018 by Bob Rehak

Day XXX since Hurricane Harvey