Extreme Growth in Flatland With Constrained Drainage

4/12/26 – Numerous human and geologic factors drive flood risk. Where they exist in combination, flood risk degrades the most.

One of the most important “lessons learned” since Hurricane Harvey has been how A) extreme population growth in B) a flat river basin with C) a constrained outlet … combine to increase flood risk.

Extreme Population Growth in Region

Worldwide, urbanization and impervious cover (concrete, rooftops) represent the single most consistent, anthropogenic (man-made) driver of flooding worldwide. And…

Extreme population growth in the Houston region has fueled the growth of impervious cover.

According to U.S. Census Bureau figures released in January 2026, Texas ranked as the fastest growing state in the U.S. last year. And within Texas, Harris County grew more than any other county, adding an estimated 48,695 people – a 1% gain. Harris County’s population now tops 5 million people.

But the areas around Harris County are all growing rapidly, too – at least in percentage terms. Upstream from the Lake Houston Area:

  • Waller County grew by 5.7%
  • Liberty County grew by 4.4%
  • Montgomery County grew by 4%.

The U.S. grew only about 0.5% overall from 2024–2025. So these counties are outliers as is Texas itself. Since 2020, Texas has added more residents than any other state, with approximately 2.6 million new residents.

When you look closely at the numbers, there’s a clear bifurcation. Giant metro areas, such as Houston, lead in absolute population growth. But small, fast-developing fringe counties lead in percentage growth.

Why This Matters: Impervious cover

Waller, Liberty and Montgomery Counties lie right upstream from Harris County within the upstream San Jacinto River Basin. They rank among the fastest growing counties in the fastest growing state. That confirms rapid upstream urbanization in the watershed, which puts continued pressure on the San Jacinto West and East Forks.

Meanwhile, Harris County leads the nation in absolute growth. That means:

Just as concerning, to accommodate this growing population, we see floodplain encroachment throughout the watershed. For instance, right now, Scarborough and the Texas General Land Office are trying to develop more than 5,000 of the most flood-prone acres in southeast Texas near the confluence of Spring Creek and the San Jacinto West Fork.

Scarborough Area in center from FEMA’s Flood Hazard Layer Viewer based on pre-Atlas 14 data.

Land that once might have been dedicated to parks for flood control has become too valuable for that. Land owners want to catch the development gravy train.

Crossing at the Commons of Lake Houston Floodplains and General Plan
Proposed Signorelli Development “Crossing at the Commons of Lake Houston.” Dotted lines represent floodplains and floodway. Original residents were promised this would be land for recreation. Signorelli fought the City of Houston all the way to the Supreme Court of Texas for ten years for the right to build on this land.

Fragmented Governance Complicates Growth Factor

Fragmented governance also complicates the issue. Take, for instance, the Scarborough land that borders Harris County. If the land gets developed, Harris County would face increased flood risk while Montgomery County would reap benefits from expanding its tax base. So, the two counties have opposing interests.

Another example: Montgomery County gives tax breaks to sand mines along the San Jacinto that help fuel all this upstream growth. But sediment from those same mines washes downriver and gets deposited in the headwaters of Lake Houston. That sediment reduces conveyance and increases flood risk for the people in Harris County.

Infrastructure Bottlenecks and Peak Stacking

Drainage from all this extreme upstream growth feeds into a bottleneck with a constrained outlet – Lake Houston.

Drainage from the 2,500 square mile upper San Jacinto River Basin all flows through Lake Houston.

And extremely low gradients throughout this funnel means water moves slower and lingers longer. That increases the chances of peaks from different tributaries stacking on top of each other.

And finally, other bottlenecks exist, too. Like the FM 1960 Causeway and the Lake Houston Dam with its four small gates. They have a combined release capacity of just 10,000 cubic feet per second – one fifteenth the release capacity of Lake Conroe’s gates.

For More Information

See the Lessons Page of this web site. It condenses the major lessons learned from researching more than 3,000 posts since Harvey into a sort of quick guide.

Posted by Bob Rehak on 4/12/2026

3148 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.