When do human interventions in water availability pose serious problems to freshwater accessibility?
The two major events where human intervention into freshwater supplies limits availability are pollution and waste. Pollution is an obvious case. The dumping of trash or waste into a water system (rivers, lakes) can quickly add up and cause a freshwater system to be tainted from human use. And it does not have to be an obvious pollutant like sewage to have a negative effect. Seemingly harmless industrial effluent can be detrimental. An example is the dumping of excess phosphate salts through the use of detergents into the Great Lakes in decades past. These phosphates served as a food source for algae and caused their populations to bloom uncontrollably. Limiting the phosphates in the wastewater helped reduce this problem.
The other major freshwater factor is simply humans wasting it. We have become more wise about water usage with things like low flow toilets and water restrictors in faucets but in years past water was used without any thought to its conservation. One major waste of freshwater is the watering of lawns. It comprises a surprisingly large percentage of home water use in the summer (I have read as high as 50% in some publications) and if done during the peak of the afternoon heat much of it is lost to evaporation. So coming up with better alternatives than traditional lawn watering helps to build up a surplus of water for dry summer months.