The Niobrara flows through a section of Nebraska known as the Sand Hills. This is quite literal. The hills and cliffs are sand and you can see the fallen remains as the angled slopes at the base of this cliff.
I have a National Geographic book here that lists scenic highways and byways in all the states and the only one they give Nebraska is the Sand Hills. Personally I think there's other places they could have added but at least they're right about the Hills, and your photo discounts those who think that the central plains states are nothing but flat and featureless. 26 miles, you said? That's a fair clip; I bet yer butt was sore after all that, even if you got out and stretched periodically. Living in Sidney, my son has so far tubed a small portion of the Platte and the Niobrara and fished McConaughy. Any other rivers worth traversing down there?
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The 26 miles is only the distance from the dam to Rocky Ford. You can go further. We once went about 32 miles down, but that last 5 wasn't fun in canoes. Lots of channels through sand bars. It felt like you were running a maze that was impossible to back track. But the Sand hills and the river valley continue all. I suspect it is about 100 miles.
The other river I've been told is good is the Dismal. It actually has white water. I want to try it, but never made it yet.