"If I recall correctly, the three Loup River branches were tiny creeks though were I was, along Hwy 83. I assume they might pick up volume as you move downstream."
Yes they do. But the one thing you loose as you go down stream is the wildness of the river. As these 3 rivers get bigger, and merge into the one Loup River, they go into more developed farming rather then ranching country. Lots more cabins along the river, and broken slabs of concrete thrown along the banks for 'stabilization' purposes. Some stretches have very steep 'levee' like banks... uper portions of all of these rivers is MUCH better canoeing.
Dismal river is the fastest river in nebraska. As you stated, it is VERY winding, and has the added hazards of low bridges (must portage a canoe around), and crossign barbed wire fences. NOT for novices. In one of my first posts on here last winter, mentioned the slope/gradient/current speed of the river, and most on here found it to be quite quick/steep.
"The Sand Hills region was interesting to drive through. Very windy, with nice rolling terrain. Not many trees though, mostly scrub brush. I would bet you could do a trip down either river and probably never see anyone."
When I was born, my parents lived on a ranch south of Ainsworth in the Sand Hills, along the Calamus River, also a good one to canoe. These hills are a very unique geological specimine. Actually they are grass covered sand dunes like you'd see in the Sahara, left over from the last Ice Age! The ground water is very shallow. Many of your valleys are filled with meadows where the groundwater table is actually just at the soil. When it rains, they'll have standing water, and as the ground water table lowers, so does the water level... My dad used to talk about digging a post hole (2' deep), and having it fill up with water by the time you walked back to the truck to get the post! Takes a special type of post to stand up to being submerged. Also, the freezing of the ground every winter would cause these post to pop out of the soil like zits on a teen age boy

Coming from a cattle raising farm family, this is "God's Country" as my dad would always say! Apparently this is a subjective descriptor of geography!