Houston is sinking. Slowly, to be sure. But it is sinking - into the underground hollows left by overpumping to quench development's thirst for water.
Cheap and abundant ground water has been the key to development here, explains Jeff Strause, a hydrologist with the US Geological Survey. It remains relatively cheap and abundant, but to pump that water too rapidly reduces the pressure that supports layers of clay between layers of sand, he says. This can cause clay layers to collapse and compact, creating at the surface irreversible depressions.
The settling of surface ground in this bowl of depression reached a crisis level during the last decade, when eastern portions of the Houston metropolitan area slid into the Houston shipping canal at the mouth of Galveston Bay.
The Harris-Galveston Coastal Subsidence District (HGCSD) was set up in the mid-'70s to deal with the problem, which has become more apparent now on Houston's western edge, the focus of its recent building boom.
The city sank four feet between 1906 and 1978, and at present rates of pumpage and growth, it will sink another four feet in the next 20 years, says Mr. Strause.
Subsidence is an irony as big as the public policy problem it poses for this city of impulsive and often uncontrolled growth.
There's no shortage of ground water here, says Strause. But in effect there is a shortage, because there is a limit to how fast it can be pumped.
Subsidence, which usually causes flooding problems rather than much structural damage, is a problem common to development and is found in such places as the San Joaquin Valley and Long Beach in California, south-central Arizona, Mexico City, and Tokyo.
''Ten or 20 years ago people really didn't believe ground water (pumping) caused subsidence,'' says Strause.
Now, though, as subsidence appears to accelerate in proportion to growth, Houston must look beyond the immediate rewards of rapid expansion and consider controls on it, determine whose responsibility it is and what alternatives there are to ground water, says Jim Blackburn, a Houston attorney who specializes in environmental affairs and the author of the book ''Texas Law of Drainage.''
As Strause explains it, ''There's no way you can stop using ground water.
''There's plenty of it that can be used safely, but it's a matter of spreading pumpage out properly and not pumping continuously. Ultimately someone has to decide how much can be pumped.''
Mr. Blackburn says someone must shoulder that responsibility now.
The new high-rises of west Houston are hardly leaning towers today, but one needs only look to Baytown, a peninsula jutting into, and now under, Galveston Bay to recognize that Houston has a serious subsidence problem.
Here, skeletons of the Brownwood subdivision's waterfront homes stand door-hinge deep or deeper in the salt water of Galveston Bay.
Blackburn doesn't suggest that these drastic conditions are in store for the newer west side of Houston, but enough residents are flooded when it rains that he thinks the problem should be addressed now.
Although HGCSD officials did not respond to repeated requests for an interview, printed reports from the district indicate that officials have not stopped issuing well-drilling permits in the growing western side of the city, nor have they any authority to require western water users to hook up to eastern surface water supplies.
The reasoning is that the effects of flooding in the west have yet to be connected conclusively to subsidence, says Strause, who notes that urbanization alone typically causes more flooding.
Blackburn explains that dangers of subsidence are obvious in the coastal area because a fraction of an inch can mean substantial flooding.
He says effects in the western area will take longer to see. But he and others will ask that one of the various agencies - flood control, subsidence, public works, or water authorities - take on the responsibility of studying what steps should be taken, if any, to insure that subsidence doesn't cause more flooding.
Because of previous flooding in west Houston, many residents now scurry to move furniture at the first crack of thunder, he says.
But even if solutions were proposed today, it could take five to 10 years to implement them, he warns.
Blackburn praises the subsidence district's handling of the eastern problem. The district, which permits well-drilling, halted all new well permits.
And the Coastal Industrial Water Authority weaned industrial and municipal water users from large ground water dependence by hooking them up to the abundant, but more expensive, surface water from Lake Houston and the Trinity River located northeast of Houston.
According to HGCSD statistics, in the period since 1976 when ground water usage was curtailed (by more than half in some cases), subsidence has leveled off in the worst affected areas.
However, Blackburn and other environmentalists criticize the continued issuance of well permits in western Houston, where development has been rapid in the past five years.
There is no surface water supply in this area, and conveying surface water from the east would be a costly project - nearly politically impossible, considering that ground water is so cheap and abundant, and that the effects of subsidence in the west have not been well defined.