Teddy Roosevelt, during his epic 1913-14 trip down the Amazon's uncharted River of Doubt, sought unbounded natural frontier.

WSJ: Riding Brazil's River of Doubt

And one of the many impediments to a long-mooted scheme to use the San Juan river for a second trans- ismithian canal is that it marks the frontier between Nicaragua and Costa Rica : efforts to patrol it frequently spark tension.

ECONOMIST: Border disputes