The West Fork of the San Jacinto River is a surprisingly beautiful river (if you don't focus too much on sand pits)for the coastal flatlands near Houston. Spring and fall are my favorite times, but summer allows a swim if your game.
It is usually floatable from the Lake Conroe dam about 5 miles west of Conroe, to Highway 59 above Lake Houston near Humble, Texas. If you dont mind flatwater you could continue into Lake Houston. I estimate the river distance at near 30 miles. Flow varies from below 25 CFS during droughts (if you don't mind walking about half the trip), to well above 10,000 CFS after rains (watch the mud getting in). A canoe/kayak rental business is located on east side of I45, Between Rayford/Sawdust and The Woodlands Parkway. The owner told me near 250 CFS is optimum, as a couple of rapids exist at that rate. I find 1,500 CFS nice, it gives a flow rate of about 2 MPH, and moderate paddling yields a travel rate near 4 MPH. I like to stay on the water 4 to 6 hours, with another 2 hours for breaks exploring sand bars and looking for pretty rocks (a rarity in Houston proper).
Real time flows are available on the web, I think from USGS. The San Jacinto River Authority operates the Lake Conroe dam, and they have told me the water release rate when I called them several years ago.
Youre in metropolitan Houston area,; If you cant find what you want you either havent looked hard enough, or it don't exist!
From Houston take Interstate 45 to put in points described above (Highway 105, Interstate 45, Highway 242, Highway 1314).
Several put in and take out points include Highway 105 west of Conroe, Interstate 45 South of Conroe, Highway 242 east of Interstate 45, south end of Bennett Estates off of Highway 1314 between Highway 242 and Porter, Texas, and Highway 59 near Humble, Texas. There is also a road called McCellan Sorters that runs from just north of the Highway 59 just north of the San Jacinto River bridge to Highway 242. A subdivision on the west side of McCellan Sorters, about 2 to 4 miles northwest of Highway 59 has a boat ramp in a subdivision park on the river. The river flow usually gets slow near this boat ramp due to the backwater from Lake Housto. I sometimes leave the pickup vehicle outside this park (in case they decide to lock the pipe gate).
The river has very dense wooded banks upstream of Interstate 45. I usually start at Interstate 45 or Highway 242, and get out at Bennett Estates or the subdivision off of McCellan Sorters Road.
San Jacinto River Authority