I don’t “claim” that a rainfall line runs through Houston, it does. To the west of Houston you see a rapidly diminishing pattern of rainfall(starting at about 42 inches), and as you get out to Katy and beyond (rainfall drops into the 30s) you can see the “Southwest” beginning. By the time you get to San Antonio the rainfall is typically in the high 20’s and it keeps going down as you go west. On the east side of Houston you have much heavier rainfall (over 50 inches), different vegetation, alligators, etc, which is why east Texas was a part of the old Cotton Kingdom.
Most of Texas has under 30 inches of rainfall, and a lot of the state has under 18 inches of rainfall. Only areas east of a line you can think of as running north-south through Austin and Fort Worth consistently have over 30 inches of rain, and only to the east of Houston do you have areas with over 50 inches of rain (with the exception of one area along the coast in Brazoria or Matagorda counties, I think). Anybody who drives on I 10 from Baytown or Dayton to Sealy can easily see how the landscape changes because of changes in rainfall patterns.
Yes, much of the early settlement in Texas was along or near rivers - e.g. Brazos, Trinity, Colorado, Sabine, Rio Grande - for obvious reasons.
Actually, the main “cotton growing area” in early Texas was the Brazos River Valley from Waco South to the Gulf. East Texas was not particularly suited for Cotton farming. It is red clay hills or sandy loam with Pine forests. The Brazos River Valley is Blackland “Bottomland” and makes for great cotton farming. You don’t need 50” of rain to farm cotton.
Heavy rains at the wrong time can wipe out the crop.
I live east of Houston in Chambers County. We get 50” average annual rainfall and were never a huge Cotton producing area. A little further to the east is Beaumont which averages over 60” of rain and can easily get 70”+ in a “wet” year. That area was never a major cotton producer either. (But its great country for rice farming.)