There are two types of water in all the world -- freshwater and saltwater. Oceans, or saltwater, cover about 70 percent of the Earth's surface and account for 97 percent of our water. That means that only 3 percent of the Earth's water is freshwater, and nearly three-quarters of that freshwater is locked away in glaciers and ice caps [source: WWF].
For humans, saltwater isn't drinking water -- our bodies can't metabolize the saline and minerals. It's possible to desalinize saltwater to make it safe for drinking, but the price tag on that process is high -- five times the cost of processing water from ordinary sources. So we're pretty much dependent on freshwater to quench our thirst. And that's not the only reason we need freshwater -- we need it to grow food and we even use it to fuel electricity.
All of this is to say that freshwater sources are vital. And it's bad news that they're under threat. The statistics are alarming. Since 1970, freshwater species have seen a 50 percent decline. More than a billion people lack access to clean water, and 2.6 billion people don't have access to clean sanitation. No clean sanitation means a polluted water supply and tainted freshwater sources. An average of 4,500 children die per day due to unsafe water and inadequate hygiene [source: WWF].
Freshwater sources range from streams, rivers, lakes and ponds to groundwater, cave water, springs, floodplains, and wetlands like bogs, marshes and swamps. These freshwater havens are home to many plant and animal species -- in fact, scientists have documented over 45,000 freshwater species. There are dozens of reasons why freshwater is in danger. But what is the biggest threat to its survival?