If you are familiar with the Los Angeles River, you might be surprised that it is a natural river at all. Because today, it looks a lot more like a massive concrete ditch. In the 1930s, the L.A. River was almost completely encased in concrete to prevent frequent and damaging floods. That flood control worked, but it also devastated the river's natural ecosystem.

An estimated 90% to 95% of the river habitat has been lost. Now, this is an extreme example, but rivers worldwide have been heavily modified to suit human needs, like bringing water to agriculture or controlling floods. These modifications have huge impacts on ecosystems - destroying habitats and polluting waterways.

So, we, just as a species, want to restore river ecosystems, but there's no like, undo button for the changes we've made, and we have to balance critical human needs, like growing food and not being flooded. One powerful tool to help river restoration comes in the form of plants. Restoring native plants around the river helps the entire ecosystem regain its hold, and depending on exactly how we do it, these plants can also help the restored ecosystem continue to do what we humans need.

So, let's take a look at how to strike that balance. The technical term for an area of vegetation surrounding a river is a riparian buffer. Most of these are fairly small, less than 20 meters on each side of a river and less than 10 kilometers along its length, but as we will discuss, bigger buffers do tend to have more benefits.

These plants are crucial to providing habitats for animals who live in and around river. First of all, they provide shade, which cools and moderates the temperature of the entire ecosystem. Big swings in temperature can decrease biodiversity, and a warmer river holds less dissolved oxygen in its water.

Just 30 meters of plants on each side of a river can keep the water temperature the same as if the river flowed through a massive forest. In terms of habitat, they can provide nesting sites for birds and act as wildlife corridors for animals to safely travel through areas of human development. Larger pieces of wood that fall into the river provide habitats for fish and invertebrates, and having a buffer also means cleaner water.

Riparian buffers can essentially filter out pollution, nutrients, and sediments. In particular, this matters when your river flows through farmland or other places where a lot of fertilizer is used. Fertilizer runoff can cause eutrophication, where extra nutrients cause excess algae to grow.

This can deplete the oxygen in the water, with consequences that ripple through the entire ecosystem, but having a buffer around the river means that plants and microbes can use up the excess nutrients before they reach the water. The improvement in water quality is pretty dramatic. One analysis of multiple studies, published in 2007, found that buffers in general remove 67% of nitrogen, and wider buffers of at least 50 meters can remove 85%!

Protecting rivers from too many nutrients helps keep this pollution out of lakes, wetlands, and the oceans they flow into, but riparian buffers are complex systems, and accounting for the ins and outs is tricky. Even though they are great at absorbing nitrogen runoff from fertilizer, in the process, they can also emit nitrous oxide, which is a greenhouse gas, and they don't work for every type of pollution. One study found that buffers made from a particular willow shrub didn't do much to block runoff of glyphosate - that's a widely used herbicide sold under the name Roundup.

It's a good reminder that plants are not some magical solution that can stop anything we throw at a river. We need to minimize pollution in the first place, and we need to understand how restoring buffers will achieve our desired goals, and of course, not all buffers are the same. Factors like how long and wide they are, how far upstream they're located, and what plants they include all change their effectiveness, but if you make a bigger buffer, you could take land away for other uses like farming.

So, how do we balance human needs with the ecosystems we aim to restore? Sometimes, it can be as simple as which plants we choose. Some profitable crops, like the American hazelnut, can be planted as part of a buffer.

This means they allow the land to be used for human purposes while improving the river's habitat. The nitrous oxide problem indicates that we need more research on how buffers emit and absorb greenhouse gases, but according to a 2010 study, buffers still take more carbon out of the atmosphere than they give off. So, restoring buffers, helps us help the climate, as well.

What is clear, is that the benefits of riparian buffers outweigh the downsides. Restoring river ecosystems isn't easy, but rebuilding the plants surrounding waterways is a great first step. If done right, it can lead to a huge number of changes that percolate through to many other aspects of the ecosystem.

The L. A River still has a long way to go, but a 2016 feasibility study proposed restoring the area around an 11-mile section. The goal is to complete the majority of the work by 2028.

Other restored rivers are already doing better, like the Green River in the Puget Sound region of Washington state, where the removal of a levy has helped restore the forested floodplain and provide habitat for wild salmon. We've made mistakes in trying to force rivers to serve humans at the expense of our environment, but adding plants back to river ecosystems can help us start to correct these mistakes while still meeting our own needs.

This video was made in collaboration with BBC Earth. Their initiative, Our Green Planet, has three main goals: to raise awareness of how awesome our planet's green ecosystems are to connect people with a plant's critical role in biodiversity, and to spread these stories of positive change makers around the world.

Thanks again to BBC Earth for collaborating with us on this video. Head over to bbcearth.com/our-green-planet to find out more, and thanks for watching.