Trout streams are not evenly nor randomly distributed on the landscape and bedrock geology and glacial history are the two best explanatory variables. Then factors such as human impacts can be responsible for declines or even extirpation (local extinction) of trout. Impervious surfaces - those that do not allow groundwater to infiltrate the soil like roads, houses, and other development - increase stream water temperature and reduce groundwater recharge. And small changes in the amount of impervious surfaces may lead to the decline or even loss of trout in Wisconsin and Minnesota (and other places; Wang et al. 2003).
A quick glance at a map of Wisconsin's trout streams is pretty revealing, more so if you know a bit about the geology and glacial history of the state. There are some pretty obvious features like the Karst geology of the Driftless Area containing an abundance of trout streams and the big expanses void of trout streams - the region between the Wisconsin Glacier extent and the Driftless Area and a rather large part of Eastern Wisconsin inside of the Green Bay Lobe of the Wisconsin Glaciation. There are some interesting political boundaries - like Menominee County and some other places where county lines change regulations and potentially trout stream designation. And as mentioned previously, development is likely responsible for the loss of some trout streams.
If you are a Midwestern trout angler, you certainly have some understanding of the Driftless Area that escaped the Wisconsin Glaciation, named for the state the it had the greatest impact on. The Wisconsin Glaciation lasted from about 75,000 to 11,000 years before present. Previous glaciations are more difficult to see and understand as they are old, often buried by features of the Wisconsin Glaciation, and have had time to be eroded away. However, they have also impacted Wisconsin and parts of the Driftless Area.
It is probably important to define "drift" here as well...drift simply refers to materials moved by glaciers. In much of Wisconsin, drift is evident as it creates prominent landscape features such as moraines, drumlins, kames, and eskers - which are all "hills" created from glacial drift. In Wisconsin, two types of moraines are important. Terminal moraines are, as the name suggests, where a glacier ended or terminated. These represent the further extent of the drift moved by that glacier, in this case the Wisconsin Glaciation. The Kettle Moraine is an interlobate moraine, where the drift moved by two different lobes of a glacier came together. The word kettle comes from the depressions, the voids created by large ice chunks within the drift that often fill with groundwater to become lakes or ponds that formed within this moraine.
Ultimately, the answer for why geology and glaciation matters when it comes to trout streams has everything to do with groundwater and the creation of springs which are basically areas where groundwater becomes surface water. And even for our "freestone" rivers, springs are important for delivering cold water and keeping these streams and rivers as trout water.
Springs - at least those large enough to be inventoried for the spring survey by the Wisconsin Geological and Natural History Survey - are formed most commonly by one of four geologic processes but the two most common are springs associated with moraines and those associated with the layered sedimentary bedrock of the "Driftless Area". The other two geologies are the porous sandstone rock primarily in Dane County and carbonate aquifers associated with the Niagara Escarpment. Wisconsin's largest spring is in Door County with a discharge of over 18 cubic feet per second (cfs; over 8,000 gallons per minute) but it comes out of the ground at 16.8*C (about 62*F) which is quite warm compared to most springs that enter trout streams. Most springs in Wisconsin run at between 9 - 11*C (approximately 48 to 52 *F; source: Springs in Wisconsin).
Source: Wisconsin DNR; Inland Trout Management Plan, 2020-2029
The springs in the Driftless Area are pretty simple to understand. Most occur where there are fractures in the sedimentary bedrock, typically where rocks of two different types and ages intersect. Many are rheocrene springs meaning that they discharge to a defined stream channel (at times, creating them) and the rest are fracture or contact springs meaning then come from fractures in rocks or they groundwater is intersected by a stream or lake. And it is easy to see why the Driftless is home to about half of the surveyed springs based on the depth to bedrock map (below). The weight of the bluffs creates pressure on the water within the spaces in the sedimentary rock and without glacial till to get in the way, fissures in rocks already near the surface create an easy path for groundwater to become surface water. Hence, nearly half the springs in the state are in the Driftless Area.
From the Wisconsin DNR, depth to bedrock
The second most common type of spring in Wisconsin occurs within glaciated regions where topographic changes mean that groundwater is pushed from end moraines or interlobate moraines where they intersects waterbodies such as lakes and streams. These are the springs responsible for the trout streams of the Central Sands. Their distribution is quite evident in the figures depicting springs and trout streams above (and Figure 16 in An Inventory of Springs in Wisconsin).
Source: An Inventory of Springs in Wisconsin; Wisconsin Geological and Natural History Survey publication
Springs are the life blood of Wisconsin trout streams and ponds and are dictated by geology and glacial history. There is so much more to know and the Wisconsin Geological and Natural History Survey's inventory of springs in Wisconsin provides a trip into the abyss if you're willing to travel there (see sources below) if the above leaves you wanting more. The springs inventory is such a cool project, I hope you search on for more information. Cheers!
Sources:
Inventory of Springs in Wisconsin and link to appendices (Wisconsin Geological and Natural History Survey publication)
Wang, L., J. Lyons, and P. Kanehl. 2003. Impacts of Urban Land Cover on Trout Streams in
Wisconsin and Minnesota. Transactions of the American Fisheries Society 132:825–839. (Download link - may require subscription)