I don’t think rivers are that big a problem, you just move the fence up the bank a little bit. But there may be some places where cliffs, canyons and such would make it problematic. If you built one, you might have to have it divert away from the border in places.
The Israelis are security conscious, are they not? And much more skilled at achieving it than we. And they rely on armed outposts to control the West Bank of the Jordan. There is no fence on either side of the Allenby Bridge.
Fences and rivers are incompatible. Especially in semi-arid territory.
I'm all in favor of securing the border. But a fence isn't a workable solution along the Texas-Mexican border. It would be a burden to Americans...and a boon to the Mexicans.
What's the good of that?