Not all prison farms were, or are, along the brazos River.
At the Rusk State penitentiary system in East Texas, convict gangs produced tobacco and harvested timber from the Piney Woods region for the prison's sawmill. Fruit, vegetable and livestock operations also were carried out. For a number of years, the prison was a large pig iron producer; I dont know how the work gangs were constituted for the iron, brick and manufacturing operations. Prison operations at Rusk closed in 1931.

The Huntsville area in Walker and adjacent counties has the largest concentration of prison farms. The prison farms here were producing significant amounts on cotton by the time of the Civil War. There are numerous prison farms, some of which are 50 or more miles from the Brazos and closer either to the Trinity or Navasota Rivers. Much of Walker County is controlled by the prison system and much of the rest by the National Forest Service. Sugarland and Oyster Creek prisons and their farms are here.

Towards the Gulf, Brazoria County, near the mouth of the Brazos, has the Darrington, Retrieve, Clemens and Ramsey Prison Farms.

Jester State Farm near Richmond, close to the Colorado River,
produced both cane and bricks. It held over 5000 acres in the 1920s but I don't know the present holdings or products.

Grimes County has two large prison farms, near Navasota.