Attacking other infrastructure was probably a war crime as well (think bridges or power plants) but there can at least be arguments made that these are dual use facilities. Militaries use bridges, military production uses generated electricity. If the US destroyed all the bridges in Iran or shut off all the power, as Trump has threatened numerous times, I would definitely consider it a war crime. However, lawyers could try and argue that because of military use, these are allowable targets.
However, a reservoir serving a civilian community is unarguably a crime. The military will get its water somehow, the civilians will suffer. And note, this is one of the hottest and driest places on the globe. Academic data for the Sirik region confirms that summer temperatures routinely peak between 45°C and 48.5°C during the warmest months. That is over 115 degrees Fahrenheit. That is
comparable to Death Valley in California.