Buildings, trees, hills, etc. are seen as bumps by the atmosphere. These objects are obstacles that impede the flow of air. Compared to land, river water is a relatively flat surface. So, the faster wind speeds you measured as you approached the river could have been the true “unobstructed” wind!

I saw a couple nice explanations, certainly a strong current could increase the wind speed by an upward transfer of momentum.

I’d like to offer another idea possible explanation which is that we tend to observe the wind in general is faster over water than land because of friction.