There are several factors that affect water availability. Natural factors include amount of rainfall as well as ground structure and composition. Humans also do a number of things that affect water availability such as using up groundwater faster than it is replaced, building man-made lakes/reservoirs to store water, and using large amounts of water for irrigation of lawns and crops.
Negative Impacts of Reservoirs
Several negative impacts of dams as well as some solutions are described in the video below. As you watch this video, keep in mind that nearly all of the dams in the world are used to store water for people and removing them would pose a significant challenge to many areas.
Overuse of Groundwater
If groundwater is overused, it will run out. The Ogallala Aquifer in the Mid-West is an example of an aquifer that is being used much more quickly than it recharges. Watch the video to learn more.
The Colorado River
The Colorado River is used by several states in the driest part of the Western US to supply water for drinking, irrigation, industry, and power generation. The combination of water use by areas not in the watershed of the Colorado River Basin and large amounts of evaporation from the many reservoirs along the river cause the river to rarely make it to the ocean. The video below explores how people are trying to change this.