NARRATOR：

Listen to part of a lecture in an environmental science class.

MALE PROFESSOR：

I'd like to continue with the topic of managing water resources, but I want to focus on a particular case, a—uh, an example of water management that's made us reconsider the methods we use when we make these decisions.

So let's look at what's happening in the Colorado River basin.

The Colorado River basin is a region in the southwest United States.

Seven states rely on the Colorado's water, and as you can imagine,as the populations of these states began to grow, it became clear that a system to distribute, ah, to make sure each state got its fair share of water, some kind of system had to be created.

And in 1922, a water-sharing agreement was made.

Elizabeth, you have a question?

FEMALE STUDENT：

Well, how exactly do you figure out how to share a river? I mean, you can't, like, cut it up into pieces.

MALE PROFESSOR：

Well, let's start with the first step. And that's trying to figure out how much water, on average, flows through the river each year.

Now, researchers had started gathering data on water flow back in the late 1890s, using instruments they placed in the river.

When the 1922 water-sharing agreement was made, there were about twenty years of data on water flow available.

The average annual flow was calculated, and, well, the agreement was based on that calculation.

The same basic agreement is in effect today.

FEMALE STUDENT：

Wait. That was all the data they had? And they based their decision on that?

MALE PROFESSOR：

Yes, and we'll see why that was a bad decision in a moment.

OK. As decades passed, it became clear that measuring river flow was much more complicated than we had thought.

See, a river has periods of low flow and periods of high flow,and this wasn't taken into consideration when the 1922 agreement was made.

In the 1970s, the population of the area was rising,while the amount of water flowing through the river seemed to be falling.

By this time we had-what-a hundred years of recorded data to look at?

That's still a pretty short time for an ancient river.

To get more data, we looked at a different source... a source that was able to tell us about hundreds of years of the river's history. Tree rings.

OK, let me explain.

You probably know that we can determine a tree's age by counting the rings on a cross section of its trunk.

Each ring represents one year of the tree's life.

So if you know the year the tree was cut, you can count inwards and date each ring all the way back to the center.

You can also tell how much moisture the tree got during each of those years by looking at the width of the rings.

A wide ring means plenty of water, while a narrow one indicates less.

Fortunately for us,certain areas of the Colorado River basin are home to some very old trees, some 800 years old and older.

Researchers can drill core samples, uh, basically get a cross section of a tree without having to kill it, look at the rings, and get a picture of what the climate was like in the basin for each of the tree's years.

Well, the results tell us something we wouldn't have known without this data-that over the past 500 years or so, the Colorado River basin has experienced severe droughts, some worse than any we've ever recorded.

They also show that the early to mid-1900s- when most of the data that led to the water-sharing agreement was collected- well, this was the wettest period in the past 400 years.

Well, obviously, had water management officials known then what we know now, the 1922 agreement would have been handled differently.

But today, we can use the past to help prepare us for the future.

With the demand for water in the basin states increasing and with the real likelihood of lower flows in the river- if history is our teacher-we can develop innovative methods of water conservation and reevaluate how water is distributed.