BOSTON -- The Head of the Charles Regatta begins Saturday and the hundreds of thousands of athletes and fans who grace the Charles River this weekend may notice the river is in the midst of a 10 year cleanup.

NewsCenter 5's J.C. Monahan said that while the river is much healthier than it used to be, environmentalists are turning to new technology to solve an old pollution problem.

When it comes to recreation, the Charles River is one of the busiest in the world. The Environmental Protection Agency estimates that 20,000 people use the river each day. Despite the river's popularity, it's not always clean enough to use.

Seven years ago, the EPA began its "Clean Charles 2005" initiative. The goal is to make the river suitable for swimming and fishing in 2005.

"I think the Charles has made some dramatic recovery since we started in 1995," EPA spokesman Bill Walsh-Rogalski said.

The river was still highly polluted when then Gov. Weld took the plunge to celebrate the river's protection bill. Since that time, the river's annual report card has jumped from a "D" to a "B". The river was once only clean enough for boating 39 percent of the time -- today it's at 95 percent, Monahan said.

"I think we're reaching a point where each increase becomes harder to attain," Walsh-Rogalski said.

One problem area is just below the Watertown dam where a bulk of the river's contamination enters.

"There are some lingering problems and questions about sources of contamination," U.S. Geological Survey spokesman Peter Weiskel said.

It's a mystery Weiskel will use DNA testing to solve -- a first for the river and a leap into high-tech water testing. He hopes the DNA results will point to the source of the problem.

"It's a powerful new technique that will help get toward the management question of how do we eliminate that last 5 percent or so of the time the river is violating standards," Weiskel said.

To help keep rivers clean, pick up after pets and keep contaminants, such as motor oil or fertilizer, from running into storm drains. All these sources of bacteria often end up in our rivers.

"To the extent that you want to use that water or you want your kids to be able to swim in the lower Charles, it's important that you maintain high standards in the way you behave, in the way you eliminate contaminants to the Charles," Walsh-Rogalski said.

We'll have to wait and see if the DNA results lead to a cleaner river. In the meantime, the pollution problem may keep people from taking the plunge.