Everyone knows the Los Angeles river. Even if it’s not part of your neighborhood, the concrete-lined channel is familiar worldwide, because nothing else in the movies or television better depicts “urban wasteland” than this drain.

But soon this stark, 51-mile (82-km) waterway may symbolize something else: tough choices amid water scarcity.

California’s historic five-year drought has prompted the Los Angeles region to look at using the river as a water supply – a role it has not served in over a century. Wastewater treatment plants, whose discharges now provide most of the river’s flow, may divert some of their treated effluent to groundwater recharge projects. Other plans are percolating to capture storm runoff before it reaches the river, both to recharge groundwater and improve water quality.

All these actions would leave less water in the river for two other important and fast-growing movements: improving wildlife habitat and expanding recreational opportunities.

A new report from the University of California, Los Angeles Grand Challenges program puts the conflict in stark terms: if all the currently envisaged stormwater capture and groundwater recharge projects go ahead, the L.A. river will be completely dried up, leaving no water for wildlife and recreation.

With careful choices, however, the 146-page report suggests, it may be possible to achieve all these goals.

To learn more, Water Deeply recently interviewed Katie Mika, the lead author and a postdoctoral scholar who grew up in the Los Angeles area.

Water Deeply: What do plans for the river look like?

Katie Mika: There are several plans to put in a variety of water-capture projects. They’re projects that could happen but won’t necessarily happen.

There’s a water capture master plan that the L.A. Department of Water and Power did. More than 100,000 acre-feet of stormwater capture projects have been identified in the region. Some are as large as identifying places to put in new spreading grounds [where water is retained to allow it to seep into the ground] to infiltrate stormwater into the groundwater basins. Then there are projects down to the parcel scale, using low-impact development approaches to capture or reuse stormwater on individual building sites.

Water Deeply: What are some examples of these projects?

Mika: One example is the Broadway Neighborhood Greenstreets Project, with a cistern that could capture water from upwards of 100 acres (40 hectares) of land. The projects that have been identified cover a huge range of scales, from a single dry well on one property up to many acres of spreading grounds.

One of the projects that’s definitely happening is at the Donald C. Tillman water reclamation plant, which discharges into the L.A. river. The city is looking to use up to 30,000 acre-feet of treated wastewater there to recharge groundwater via spreading grounds. That’s one of the biggest impacts that could happen on flow.

Water Deeply: Are there proposals now to start drawing surface water from the river again?

Mika: That isn’t part of the plan. One of the things that could be done is capturing a lot more stormwater before it gets into the river. And people are looking at places that we can divert water from the river to other places, like spreading grounds or treatment plants. So those still would likely be an indirect use of water in the watershed.

Water Deeply: How much water does the L.A. river carry today?

Mika: As kind of a ballpark number, the annual average flow of water going through the L.A. river is around 274,000 acre-feet per year. That would include the treatment plant flow and precipitation runoff. There might be a couple of places where there would be a tiny bit of infiltration, but most of that flow is being discharged to the ocean. And just for context, the city of Los Angeles total water supply tends to be between 500,000 and 600,000 acre-feet per year.

Water Deeply: And how much will be left after all these proposed water capture and reuse projects?

Mika: If L.A. reuses all of its recycled water in the watershed, without discharging any of it to the river for environmental reuse, it would lead to a nearly dry river for much of the year. When we ran a scenario that had no effluent discharge to the river and all the stormwater capture programs were in place, then the flows in the river went to zero. The river was dry.

And that would have impacts on recreation and on aquatic life. We just don’t know what flows we need to support those other uses.

An example could be the urban kayak adventures, which have become popular on the river recently. Probably that wouldn’t be feasible without treatment plant flows during summer months, at a minimum. On habitat, we really need to figure out what a healthy one looks like, and what flows we need to support that habitat.

That’s the conversation we need to start having as a region. None of them are things you want to give up. They’re all really important pieces of it. The L.A. river can’t be everything to everyone. But a lot of things do have seasonal needs. And with water storage we can probably figure out a way to do everything at least some of the time.

Water Deeply: Do you think this finding will surprise people?

Mika: I think aspects of it might surprise people. I was surprised. I think it really highlights that a lot of the plans and projects that are happening are going to have impacts outside those particular projects. There’s probably been a background awareness that the flows are coming largely from treatment plants. I think this will really put numbers on it and highlight it.

It’s a 51-mile river, an 800-plus square-mile watershed, and it’s very different in different parts of the river. There are going to be very different opportunities and trade-offs in each of those places. So we have to think about those as well. The L.A. river is very complex, and the trade-offs and discussions will be different everywhere.

Water Deeply: What did the L.A. river look like historically?

Mika: The river is why the city is located where it is. It was the city’s only water source until the early 1900s. The groundwater tables were much higher also. There were a lot of artesian springs in the area. And there was year-round water.

Water Deeply: And how has that changed more recently?

Mika: Once we started pulling out a lot of the groundwater, the flows in the river got much lower year round, and now we don’t take any water directly from the river. There is still some small volume of groundwater that’s recharged from the river but there is no surface diversion from it right now.

Water Deeply: Why should we worry about maintaining flow in the L.A. river?

Mika: That’s the very question this report has laid the groundwork to talk about.

One reason is there’s actually a surprising amount of habitat on the L.A. river. On the lower end, down by the harbor, the current flows actually have led to algal mats forming that have become really important for birds. In the middle of the river, the Glendale Narrows is actually a soft-bottom habitat and there’s some wildlife there, and that’s where the recreational use is happening, like kayaking.

For the last 10 or 20 years, the focus has really been about the river as a recreational space and to improve water quality. It really is a space that can offer a lot of connectivity between regions, and it offers greenspace potential in areas that don’t have a lot of green areas in them. There are a lot of projects that have been happening through that lens.

The type of study that really needs to be done to think about the best future for the L.A. river would really ask what does a healthy habitat and ecosystem look like, and what flows do we need to maintain that.

Water Deeply: How much water in the river today is treated wastewater?

Mika: It varies. During dry weather, it can be up to about 85 percent. In winter it would be a much smaller volume. The surprising part of this is thinking about the L.A. river and its watershed from a water supply perspective, in addition to just as a drain.

Water Deeply: What is L.A.’s low-impact development ordinance and how does it affect the river?

Mika: It’s a city of L.A. ordinance that became effective in 2012. It says all new development and redevelopment projects that create or replace 500 square feet of impervious area have to capture a three-to-four-inch rain event for infiltration or reuse on site. It can provide a significant water quality and supply benefit.

If the L.A. ordinance was expanded to cover the entire L.A. river watershed, then it would be about 3,000 acre-feet of water that could be captured by 2035. Within just the city of L.A., it would be about 1,600 acre-feet. That’s not the per-year number – that could happen every time you get a storm. It’s not a ton, but it is a significant chunk. There are other cities and agencies within the watershed thinking about adopting a version of the ordinance, but I’m not sure if anyone has actually done it.

Our report is a planning tool in a lot of ways. We look at all of these various efforts, and the last 60 years of data in the river, and really try to see what effects all these different plans and projects have on the past and present of the river and what that means for planning its future.