But most their water disputes have ended with handshakes, not violence, thanks to a 1960 treaty, says environmental security expert David Michel at the Stimson Center, a Washington research group.
"The Indus Water Treaty was developed and signed largely due to the parties’ recognition of the possible consequences of failing to come to some agreement on how to manage this river that crosses their borders," said Michel.
But managing river resources may get more difficult in the coming years. Both countries are growing rapidly, raising demand for food, water and energy. And there is no additional water in the river basin.
Michel says it’s not only happening in South Asia.
"We are increasingly bumping up against the limits of available renewable water supplies in many regions of the world," he said.
On Southeast Asia’s Mekong River, plans for 12 electricity-generating dams are raising concerns for those downstream who depend on the river for their fields and fisheries.
And rights to rivers and underground water are flashpoints in sub-Saharan Africa and the Middle East.
Challenge of sharing resources
Sharing these resources will be a growing challenge as climate change shifts precipitation patterns, creating winners and losers. But that’s not all, Michel says.
"Climate change is not the only, and is not even the most important water pressure in the immediate future. Population growth is really driving the water resources challenge for the coming couple of decades," he said.
Larger populations mean more demand, but also more pollution. Michel says even contaminated water may be in greater demand.
"Water scarcity, water stress, will often drive people to use more polluted water sources. One very, very troubling consequence is the impact on public health," he said.
Waterborne diseases cost India alone more than 6 percent of its gross domestic product each year. And pollution flows downstream.
Although these pressures raise the threat of conflict over water, Michel says wars are not inevitable.
"The pie to be divided grows substantially with more cooperation. And countries - even those that are often at odds - they recognize that," he said.
So Michel says he sees a future with fewer water wars and more handshakes.