What prevents navigation on the lower Niagara River upstream from Lewiston and Queenstown?

The presence of heavy rapids on the lower Niagara River upstream from Lewiston and Queenstown significantly impedes navigation. These rapids, known as the Niagara Rapids, create turbulent, fast-moving water conditions that can be hazardous for vessels. The strong currents and abrupt changes in water elevation make it extremely challenging for boats and ships to navigate safely. The physical dynamics of the water at this location mean that only specially designed vessels, such as those equipped for whitewater navigation, can attempt to traverse this section of the river, and even then, only under very careful conditions.

Other factors like rocky ledges and water levels may also be obstacles, but it is the heavy rapids that are the primary concern for navigation safety when considering upstream travel in this region.