The issue that killed solar roadways (the covered kind, not the stupid ass embedded kind) is that people would inevitably crash into the support beams, leading to collapses. That means the structure would have to be completely over engineered, increasing costs. Plus, the dynamic pressure waves from the passing trucks and cars underneath plus the fact you need to build it tall in order to allow trucks to pass means it needs to be even stronger. Solar over a concrete river is not going to experience these problems and can be minimally constructed as a failure just leads to them falling in the river, not actually harming anyone.