Watersheds and human activity

0:00- [Instructor] This is the Chesapeake Bay. It's one of the largest bays in North America, rich with life. You'll find blue crabs, oysters, fish, and millions of migratory birds. But for decades, large parts of that bay have started to turn into something called dead zones.

0:19These are areas where there isn't enough oxygen for many fish and other aquatic animals to survive. So the question is, how can a place that supports so much life suddenly struggle to support life at all? Let's find out. What's important is that the Chesapeake Bay doesn't just collect water from the land right next to it.

0:40It's instead fed by rivers and streams flowing from a huge watershed. How huge, you ask? Well, it covers more than 64,000 square miles, stretching across six states and Washington DC. So that means the bay isn't just affected by what happens in the bay, it's also affected by what happens everywhere upstream.

1:05Every drop of water flowing downhill picks up whatever it passes over, like nutrients, pollutants, or sediments, and carries them downstream. That's the whole point of watershed, everything is connected. So let's dig a little deeper to see how our activities affect the bay.

1:25One of the biggest human influences is agriculture. Farms across the watershed use fertilizers to help their crops grow, but fertilizers don't always stay where you put it. When it rains, some of the nutrients from the fertilizers dissolve in water and flow into nearby streams.

1:47Those streams eventually feed into rivers, and the rivers carry the nutrients all the way to the Chesapeake Bay. Well, that doesn't sound too bad, right? Well, think about this. Because of the nutrients, the plant and algae start growing, but too many nutrients cause huge algae blooms to spread across water.

2:07Now, when those algae eventually die, bacteria break them down. And as bacteria decompose these algae, they use up oxygen from the water. And so, as oxygen levels fall, the fish and many other aquatic organisms can no longer survive. This is how dead zones are formed.

2:27And by the way, this isn't just happening in the Chesapeake Bay. In fact, one of the world's largest dead zones form every year in the Gulf of Mexico, because nutrients wash into the Mississippi River from farms across its enormous watershed. But anyways, human activity affects watersheds in other ways too.

2:48A main one is urbanization. As towns and cities in the watershed grow, more and more land gets covered with pavements and roads and parking lots and rooftops and whatnot. The problem is these don't let water soak into the ground. So again, when it rains, the water just runs off fast, picking up oil and chemicals and thrash along the way before dumping all of it into the nearest stream.

3:18And that eventually ends up in the bay. And growing cities also pull more water out of the ground, and sometimes that happens faster than the ground can replace it. Then there's deforestation and soil erosion. See, tree roots do an important job. They hold the soil in place.

3:37So what do you think happens when forests get cleared for farming or construction? Well, that soil isn't held down anymore. So rain can wash it straight into nearby streams as sediments, which, again, can make its way into the bay. And that clouds the water and makes it a lot harder for the fish and the plants to survive.

3:58And if that's not enough, humans have also found ways to make changes to water flow. Dams and canals physically change how water moves through the watershed, so they can block fish migration, for example. They can change the water temperature upstream and downstream, and they can change how sediments travel through the system.

4:20All of that ripples through the ecosystem. And this can actually happen on a massive scale. For example, the RLC in Central Asia was once one of the largest lakes on earth, but after rivers that were feeding it were diverted for irrigation, the lake shrank to only a fraction of its original size, devastating both ecosystems and nearby communities.

4:44Okay, all this sounds pretty grim, right, but here's the good part. Every single one of these can be reduced, and some of them can even be reversed. And in fact, Chesapeake Bay is a really good example of that happening right now. So what can we do? Well, for example, farmers can plant something called cover crops.

5:07These are crops that are grown between harvests, and what they do is they will hold the soil in place and soak up extra nutrients before they wash into the streams. And a lot of them are also planting something called riparian buffers. What's that? Well, these are just strips of plants along the banks of the streams.

5:27These act like natural filters, trapping the sediments and nutrients before they reach the water. And cities are doing their parts too with something called green infrastructure. So the idea is, instead of covering everything with concrete, they're building rain gardens, using permeable pavements, and restoring wetlands, for example.

5:54These allow more rainwater to soak into the ground instead of rushing away as polluted runoff. And forests can also be protected and restored. Keeping trees along the streams and replanting can help reduce soil erosion. All of this allows more water to infiltrate back into the ground.

6:20And finally, water conservation. For example, when groundwater is pumped more slowly, aquifers have more time to naturally recharge. And guess what? All these efforts really do work. They have made a huge difference. In fact, reports from 2023 showed that the Chesapeake Bay had its smallest dead zone in nearly four decades.

6:47That is awesome, right. I mean, it's proof that when people understand how watersheds work and work together, they can make a real difference. So, long story short, human activities can definitely harm watersheds in many ways. Think about agricultural runoff, urbanization, deforestation, changes to the water flow.

7:09They can all affect the quality and the quantity of water, not just in this particular bay but in watersheds all around the world. But we can also protect and restore the watersheds through sustainable farming, green infrastructures, responsible forestry, and even water conservation.

7:29So I think the most important idea is that a watershed is one huge connected system, something that happens many miles upstream can affect everything downstream. So that means we, individuals, communities and governments all play a key role in keeping our watersheds healthy.