Why Houston Is America's Fastest-Sinking City

On the shore of Galveston Bay, in Baytown, Texas, there's a nature park where a neighborhood used to be. If you walk its trails, you can still find old driveways leading into the water.

That neighborhood was Brownwood. It was built in the 1930s on dry land above the bay. Over the following decades, the ground under it sank by as much as about ten feet. Streets that were once safely above the water began to flood on ordinary high tides. In 1983, Hurricane Alicia's storm surge swept across what was left, and Brownwood was abandoned for good.

Brownwood is the most dramatic piece of a much bigger story. Parts of the Houston–Galveston region have sunk more than ten feet, among the most of any place in the country. And even today, when satellites compare America's 28 biggest cities, Houston still comes out the fastest-sinking of them all.

Why Houston? We'll work through three hard questions: how ground can sink when the water you pumped never left a hole, why sinking can keep going after the pumping slows down, and where Houston's sinking is heading next.

Hard question 1If the water is still underground, why does the ground sink?

Guess first: When a city pumps water from an aquifer, does it leave an empty cave behind?

No. This is the part that surprises most people. Aquifers aren't underground lakes. They're layers of sand and clay with water filling the tiny spaces between the grains. When you pump, the layer stays full of grains, and most of the water you didn't pump stays right where it is. What changes is the water's pressure.

Here's the key idea, worked out by the engineer Karl Terzaghi about a century ago. Everything above an aquifer layer, all the soil, rock and city on top, pushes down on it. That weight is shared between two things: the grains pressing on each other, and the water pressure in between them.

The weight from above doesn't change when you pump. But the water's share drops. So the grains have to pick up the difference. Sand grains are tough and barely budge. Clay is different: its grains are tiny flat flakes, and under extra load they slowly squeeze closer together. The clay layer gets thinner, and the land on top drops.

How big a change are we talking about? By 1977, water levels in some of Houston's main aquifer had dropped about 350 feet. Every foot of water level lost takes about 0.43 pounds per square inch of pressure off the water:

That's roughly ten tons of extra load suddenly handed to the grains on every square foot of clay. No wonder it squeezed.

Houston was built for this to happen. It sits on young, soft sediment laid down by rivers and the Gulf, with thick layers of clay stacked between the sands. Pump hard from a place like that, and the clay pays the price.

And it wasn't only water. Just east of Houston, at the Goose Creek oil field near Baytown, the ground sank in the late 1910s and early 1920s as oil, gas and water were pumped out, and part of the field slipped under the bay. It became one of the first places in the world where scientists documented that pumping fluids out of the ground could make the land sink.

The takeaway: pumping doesn't leave holes. It lowers the water pressure, so the grains have to carry more of the weight from above, and soft clay squeezes. Houston has a lot of soft clay.

Hard question 2Why does the ground keep sinking after pumping slows?

By the 1970s, the sinking was impossible to ignore. In 1975, Texas created the Harris-Galveston Subsidence District to cut back groundwater pumping. The region switched much of its supply to surface water from rivers and lakes. Near the coast, the sinking slowed dramatically.

We can watch it happen in a surprising place: the tide gauge at Galveston's Pier 21, which has measured the sea's height since before 1910. A tide gauge can't tell whether the sea is rising or the land is sinking; it only sees the difference. Researchers split its record into three eras:

That bump in the middle is the land sinking. Let's work out how much extra it added:

Nearly five extra inches of "sea level rise" at Galveston came from the land going down, and it faded once the pumping slowed.

Guess first: If the water pressure comes back, does the clay spring back up?

Mostly, no. Here's the clay's strange memory. Squeezed clay keeps track of the heaviest load it has ever carried. Below that level, it behaves fairly springy. Push past it, and the flakes rearrange into a tighter pattern that doesn't undo itself. When the water pressure recovers, the clay rebounds only a tiny bit. The USGS calls this kind of compaction largely permanent.

And there's another reason the sinking lags: clay is slow. Water seeps out of thick clay layers over years to decades. So the ground can keep settling for a while even after pumping eases, like a slow sponge still dripping after you stop squeezing.

The takeaway: cutting pumping works, and the Galveston tide gauge proves it. But clay drains slowly and doesn't bounce back, so the sinking takes time to stop and the lost ground stays lost.

Hard question 3Where is Houston sinking next?

If the coast slowed down, why is Houston still number one?

Because the city didn't stop growing. It grew outward. As the coastal areas switched to surface water, fast-growing suburbs to the north and west, such as the Katy area and The Woodlands, relied on new wells. The sinking followed the pumping. The hot spots moved inland, to places that had barely sunk before.

That's what the satellites show today. More than 40% of Houston's area is sinking faster than 5 millimeters a year, 12% is sinking faster than 10, and some spots exceed 20 millimeters a year.

So our prediction comes straight from the physics: wherever new heavy pumping meets thick clay, sinking will follow. Wherever pumping is cut and aquifers recover, it will slow. Regional water plans across the Houston area now aim to shift more of the supply to surface water for exactly this reason.

There's one more thing to watch: faults. The Houston region has hundreds of mapped surface faults, cracks where the ground on one side moves down relative to the other. Uneven sinking can make them creep, slowly bending roads, pipes and foundations that cross them. Remember from our article on all 28 cities: it's uneven movement that cracks buildings.

And for a flat city near the Gulf, every inch matters. When Hurricane Harvey dropped record rain on Houston in 2017, water had to find its way across land with very little slope to begin with. Ground that has sunk is ground that drains more slowly.

The takeaway: Houston's sinking follows its wells. The coast slowed down after the switch to surface water; the growth areas inland are where to watch next.

More questions people ask

Try it yourself

Take two kitchen sponges. Soak one, place a heavy mug on it, and mark the height. Squeeze the water out from the side, then soak it again. Did it return to its starting height? Now try the same with a cup of wet sand. The sand barely moves. You've just shown why Houston's clay layers sink and its sand layers don't.

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