I spent 9 years working for the fisheries department at SIUC after I finished my master's degree in aquaculture there (as well as my undergraduate degree in zoology), so I have fished nearly all of the streams down there and have spent a lot of time on the shawnee streams so I can elaborate.
Personally, I would not say that the habitat is similar in all of the Shawnee streams.
The first thing you should know about these streams is the geological region they lie in has very little limestone, it is mostly sandstone and clay. Sandstone does not hold ground water well, so the base flows of the streams can vary widely over the course of a year. I regularly saw the flow rate on lusk creek and other streams fall below 0.5 CFS throughout the summer. This leads to high water temperatures from the headwaters to the mouth at the Ohio. Many of these streams are characterized by short steep riffles or ledges and long deep pools. Nearly all streams in the area are like this, and although there are some very small springs in some areas, the volume from them is not sufficient to keep water temperatures down from June-September. Smallmouth do not compete well with spotted bass and largemouth in streams with high water temperatures, especially in areas with habitat that is less than ideal for smallmouth. Some great LMB and spotted bass fishing if you can find the right areas. I have caught LMB over 22" in Lusk and other area streams, and spotted bass to 17".
Big creek is the exception because it has several springs that keep base water flows throughout the year, and the geologic region that it drain's (Hick's Dome and Karber's ridge) has lots of limestone under the ground. Limestone dissolves, forming karst areas and this forms springs due to hydraulic pressure. Water temperatures in the creek stay below 85 degrees due to spring inflow and abundant shade and the fact the overall water volume is low and the majority comes from springs. The stream itself does not form long pools, rather short pool areas with good riffles and runs which keep the water shaded. Smallmouth, spotted bass, and largemouth are found there, but the lower water temperatures and favor the smallmouth.
The limestone also dissolves calcium in to the water which is something that smallmouth bass love. If you look at the latin name of smallmouth bass, it describes this very well. Micropterous Dolomieu (think dolomite, which is a calcium magnesium carbonate). Smallmouth bass and limestone go hand in hand. In the southern midwest, without limestone in the watershed there may not be any smallmouth.