Fabulous… I must have missed that page when I was looking through your thread. Looking at your description:
High Plains
this is your basic grass, very similar to OC rural. It will be a tall grass or prairie setting, with a very short height transition. It will also have a river system with a very small river width, but it can cut hills of various heights with tile groups to simulate bends through softer rock layers. I'm going to stick with a near-grayscale tanish color for ground, similar to that found on the road in OC rural, but with less color. Trees will consist of scrub oak and box elder, almost entirely, with the occasional buffalo berry or something sage in color to break up the monotony. In addition to the base grass color/height/mass, the area will be filled with various wildflowers and specific grasses. High plains will feature two pond types: clean and alkali. Both will be walk-in depth. Cut height transitions, as well as alkali pools, will show badlands clay rocks as their base.
What about the Big Muddy eh? From my source material…
The 'Big Muddy River’ (the Missouri) has a peculiar terror all of its own, a restless, relentlessly foaming flood, a boiling torrent for two and one-half thousand miles from highland headwaters to its union with the Mississippi; increasingly yellow-muddied and littered with turning driftwood, cursed with sand-bars and whole trees snagged upon the bottom, all deposited by the continually collapsing alluvial banks. This freshet has cut a fertile valley through the prairie - two or three hundred feet deep and from two to twenty miles wide - along which it is bumped back and forth between the alternating bluffs of sloping lawn and precipitous, variously-coloured clay cliff.