Say you're getting ready for a hike on a hot summer day. You're going into the woods, far away from home and restaurants and movie theaters and malls and even other people. What's one super important thing you should definitely pack?

Besides a compass or a map and hiking boots and the first aid kit and food and... Okay, fine, there's a lot of stuff you should pack. But the most important thing - water. Specifically freshwater. After walking a lot in the heat, nothing will make you feel better than some ice cold H2O. Why should you have to pack water though? Can't you just sip some from a stream or a river that you pass on your hike?

Not necessarily. We've already talked about freshwater so you know what it is, and that it's found in relatively small amounts on our planet. So sadly, there's no guarantee that you'd stumble across any freshwater on your hike. But does the small amount of freshwater in out hydrosphere ever change? And what happens to plants and animals in the biosphere if the level of freshwater does change?

[text: Big Question]

Time for a little refresher. You'll remember that freshwater is water that doesn't contain a lot of salt in it. You'll also remember that for a planet with a lot of water on it, surprisingly little of it is freshwater. Let's review a few freaky freshwater facts:

Most of the water on Earth is in the oceans. They're huge and also full of salt. In fact, about 97 percent of the water on the planet is salty. By contrast, freshwater is found in lakes, rivers, ponds, wetlands and streams. Only about 3 percent of the planet's total water supply is freshwater. But about 2 percent is locked up in frozen glaciers or underground, where we can't get to it. So that means of all the freshwater that's on Earth, less than 1 percent of it is actually accessible to us on the surface and also to all the plants and animals we share that surface with.

And different ecosystems have different amounts of freshwater in them. For instance, a rainforest will have a lot more than a desert. And that's okay, because some ecosystems don't need as much as others do, but all ecosystems need some.

So what happens to a single ecosystem when the amount of freshwater that's available in it changes? Let's take a look at one of the most important sources of water in the southwestern United States: the Colorado river. 

The Colorado river provides water to more than 40 million people in the states of Colorado, Utah, New Mexico, Arizona, Nevada, Wyoming and California. People use it for farming, drinking and growing their lawns and other things. But there's a lot less water in the river these days than there used to be. Almost 100 years ago in the 1920s there used to be more than 22 trillion liters of water in the river. Today, there's about 4 trillion liters less than there used to be. What can explain such a large loss of water in a relatively short period of time? In a word: us. 

Humans are using more and more of the water every year, leaving less of it for future generations. This line graph shows just how much our dependence on the river has grown over the years. The blue line shows how much water is available, starting from the 1920s into the 2000s. It's mostly going down over the decades except for a few upward spikes in especially wet years. This red line shows how much water we used each decade. This line has been going up, up, up, until it's finally met the blue line on the right. 

Some people are worried that the demand for the river's water will keep surpassing the amount of water that's actually available. And to be honest, they should be worried. Less freshwater in a certain area doesn't mean just less water for the people that live there, it means less water for the plants and animals that live there too.

The vaquita is a rare species of porpoise that lives in the gulf of California at the end of the Colorado river. It's had to try to adapt to having much less freshwater in its ecosystem over the years but it hasn't been doing so well. Today, there are fewer than 100 vaquitas left in the world. Vaquitas are just one kind of animal that live or near the Colorado river. Thousands of other fish, birds, mammals and other animals call the river home, too.

And if we take action now, we can keep it that way. When the availability of freshwater in an ecosystem, in this case the Colorado river, changes, it has a major impact on the humans and other animals that depend on it, which bring us back to our big question: Can the already low levels of freshwater in our hydrosphere change? 

Yes. And when they do, it's not great news for the biosphere. That's why it's so important that we use less freshwater than is actually available in the world. Otherwise, one day there will be less water than we need to survive. But there are lots of ways to keep that for happening and luckily, some super awesome humans have gotten together to keep our planet's freshwater levels from getting too out of balance. More on exactly what they're doing and how you can help - next time.