Maybe because that’s the most shallow water area around?
Read the article!
“ KESTENBAUM: Hintersteiner says this is a lousy place to build a bridge. The river floor is mucky. If they’d built this at a narrower part of the river, would we be in this mess?
HINTERSTEINER: No, due to the fact that they would have hit solid rock”.
Right. It isn’t always about the width of the river. The river bottom might be the best place due to large layers of stone, might be any number of things.
Bedrock