The Economist discusses the dynamics behind them. Cheap arms make it easier to kill over water, global warming is making water more scarce, and population growth is increasing demand. Does that mean wars are more likely? Not necessarily...
The main reason that people are now (or will be) fighting over water is that the institutions for managing water (property rights, prices, trades, etc.) either do not exist or are inadequate for new conditions of scarcity. In the past years of abundant water, there was no need to decide who owned what, how to divide water among others, whether it was necessary to store water, etc. But -- as water becomes more scarce -- these institutions are necessary to avoid fighting over too little supply and too much demand. As I discuss in my dissertation (chapters 3, 4 and 7), institutions for managing water do not just drop into place -- they take effort to design, need to reflect local conditions, will evolve, and create winners and losers.
Bottom Line: Under the status quo of "no rules", water wars are indeed likely. We can avoid them by designing institutions that reflect local conditions, guarantee some level of equity (e.g., per capita water rights), and allow for the efficient distribution of remaining water. I discuss how water markets can accomplish those goals in many prior posts.
Memorial Day Bottom Line*: Clauswitz said that war is politics by other means. As we consider the sacrifices of those who have suffered in war (soldiers and civilians, families, friends and communities), let's also remember and condemn leaders who have abused their power in waging war. Wars over water are between one group claiming a right to keep what another claims a right to use, and water wars often arise from the same leadership failures. As usual, it only takes one idiot to start a war, but he often finds another idiot to fight with. And the innocent suffer.
* Yeah, it's serious, and yeah, it's politics, but we should think heavy thoughts on Memorial Day. The Fallen deserve gravitas.