GWEN IFILL:There are warnings about the risk of contaminated waterways, and the dispute continues between anti-doping officials and the chief of the International Olympic Committee over Russia's participation.

That said, there's excitement around the actual competition.

Jeffrey Brown got a preview of U.S. medal hopes with Christine Brennan, sportswriter and columnist for USA Today and ABC News.

JEFFREY BROWN: Christine Brennan, welcome back.

CHRISTINE BRENNAN, USA Today: Thanks, Jeff. Great to be here.

JEFFREY BROWN: So, there's a certain point we talk about all the problems, but a certain point, let the games begin, right?

CHRISTINE BRENNAN: Exactly. It's time.

JEFFREY BROWN: So, let's talk about athletics here.

Often, Summer Olympics, Americans, and we have talk over the years about especially gymnastics, swimming and other things.

Start with gymnastics, another big year coming.

CHRISTINE BRENNAN: Oh, absolutely.

In fact, it may be the biggest year ever for USA gymnastics, especially the women's side, Jeff, which is saying something, because U.S. women have become the dominant gymnasts in the world, going back to the days of Mary Lou Retton, Shannon Miller, obviously Gabby Douglas.

JEFFREY BROWN: Just last time.

CHRISTINE BRENNAN: Right. Exactly. And she's back. She did make it and will be a steadying influence, I think, and helpful to the next big names.

So, in this list of names I just mentioned, there is another one coming, maybe the best of all, Simone Biles. She has won everything.

JEFFREY BROWN: Maybe the best — she is coming in already talked about as one of the best ever. Right?

CHRISTINE BRENNAN: Exactly.

And, in fact, she's so good that everyone just is basically handing her the gold medal for the individual all-around. Team gold medal should be the United States', no problem. And then…

JEFFREY BROWN: So, no pressure at all, right?

CHRISTINE BRENNAN: Well, and that's the point, though. It's a great point.

JEFFREY BROWN: Yes.

CHRISTINE BRENNAN: She did stumble at the Olympic trials.

Everything was set. She was going to make the team. And she had a couple uncharacteristic bobbles and a fall. And, as you know, from watching sports, as everyone at home knows, just when you think you know it's solid and it's guaranteed, is there going to be a crack here?

I think she is so good and so strong, and because she's won world title after world title, the top gymnast in the world the last three or four years, I would be stunned if Simone Biles has a misstep. But that's a tall order. And it's also over a week-and-a-half basically of events, and we will see how she handles that pressure.

JEFFREY BROWN: OK.

Swimming, return of two big stars, right?

Michael Phelps, to my amazement, I read it's his fifth Olympics coming back.

CHRISTINE BRENNAN: Right.

Yes, he was a 15-year-old in 2000 — most people forget that — in Sydney. He's now 31 years old. He's a dad, and he says he has cleaned up his act. He's had issues of course with drunk driving and other things. This is it. It's his last gasp.

JEFFREY BROWN: He says this is really it.

CHRISTINE BRENNAN: This is really it, as opposed to the other time when he was retiring the last time.

But Phelps is still at one — certainly the best in the United States at what he does, the 100 butterfly, 200 butterfly. Whether — the key question, Jeff, is whether he's the best in the world. And his times in Omaha at the Olympic swimming trials a month or so ago were not the best in the world. They were not his best.

So, the question is, for Michael Phelps, can he get better, can he get faster? Otherwise, while we just expect him to win everything, it might be a little bit of a surprise. Maybe he will win the 100 fly, which is an incredible achievement, 200 fly. But the reality is, there is a lot of competition from around the world, and these guys would love to beat Michael Phelps.

JEFFREY BROWN: And on the women's side, Katie Ledecky.

CHRISTINE BRENNAN: Oh, Katie Ledecky is gold.

Now, that's — I think if you said to me, what would be the most surprising to happen at the Olympics in terms of a U.S. athlete, it would be Katie Ledecky not winning.

JEFFREY BROWN: Really?

CHRISTINE BRENNAN: She should be five seconds, six seconds ahead in the 400, 11 seconds ahead in the 800 freestyle. She has also got the 200 free. And then she would have the 4×200 relay.

So, if everything goes well for Katie Ledecky, it would be four gold medals, the most for any American, I'm sure, at these Games in terms of swimming, and she will really come out of the Olympics — if all that happens, she will come out of the Olympics as the swimming star of the Games for the U.S.

JEFFREY BROWN: OK.

So, if we look — now I want to switch to track and field. And there the doping scandal has an impact. But start first with Usain Bolt.

CHRISTINE BRENNAN: Yes. He is back, right.

He is trying to do a triple double.

JEFFREY BROWN: A triple double, yes.

CHRISTINE BRENNAN: We hear that all the time in basketball.

Here's the triple double in track and field, Jeff. It's winning the men's 100 and the men's 200 in three consecutive Olympics. Beijing, he did it. People remember how he finishes the race, by turning around and jumping up and down, and looking at his competitors, and dancing while he's still running, and has these incredible times, and then he did it again in London.

So, here he is. He will be 30 years old on the day the Games end. That's not exactly a spring chicken.

JEFFREY BROWN: Has had some injuries.

CHRISTINE BRENNAN: And he had a hamstring pull just a few weeks ago. He says he's healthy. We will see.

JEFFREY BROWN: Now, the Russian doping scandal, we know has an impact already in the track and field, right?

CHRISTINE BRENNAN: We do. Yes.

JEFFREY BROWN: And what impact would it have?

CHRISTINE BRENNAN: Well, I think it's a big one.

Here's the reality. The Russian track and field team is banned, except for one lone long jumper. So, the fact that they were second in the medal count in London, Jeff, in track and field, they had won seven gold and they won 16 medals overall.

And those medals now are gone. They are just not going to be able to win them. So, while I personally believe Russia should have been banned completely, the entire team, for state-sponsored, government-sponsored systemic doping, the likes of which we have not seen since East Germany 40, 50 years ago, the reality is Russia is in the Games.

But track and field is where the federation said, no, we're not going to allow this cheating system, this incredible, diabolical system to get into our sports. So, the track and field federation kicked out the Russians. And that's where you will see, I think, a significant impact in terms of the lack of Russian medals, obviously.

And the U.S. and other countries will benefit from that.

JEFFREY BROWN: Speaking of medals, right, let's look at just some big picture here in our last minute.

More women competing and medal count, give me a look at the big picture when you watch these Games.

CHRISTINE BRENNAN: Right.

Jeff, we called the 2012 U.S. team Team Title IX. Well, we can call this team, the 2016 team, Team Title IX as well, the most women ever by any country being sent to the Olympics. The U.S. is sending 292 women out of a team of 555.

That is all about watching that law, Title IX, signed in 1972 by Richard Nixon, work its way through our culture. And the power of U.S. women will be extraordinary, team sports, dominating basketball, maybe soccer, water polo, others, the rowing.

And then as far as medal count…

JEFFREY BROWN: The basketball team hasn't — you were telling me before, right, hasn't lost since…

CHRISTINE BRENNAN: The women's basketball team hasn't lost since 1992.

JEFFREY BROWN: Yes.

CHRISTINE BRENNAN: Yes.

And one of the reasons people don't know is this is because they're so dominant, that the media tends to go, oh, give them the gold medal and move on to other sports.

JEFFREY BROWN: Right. They won again.

And women's soccer, of course, will be a big deal as well. And then, as far as the medal count, the U.S. should do as it usually does at the Summer Games and dominate, especially with Russia being down, understandably so, correctly. I think the U.S. will do very well.

China will be there. It will be interesting to see if Great Britain, which had a great Games, home Games, four years ago, if they can keep that up and continue to win medals in sports where no one expected them to do that in 2012. Can they keep it up now when they go to Rio in 2016?

JEFFREY BROWN: Christine Brennan, enjoy yourself down there.

CHRISTINE BRENNAN: Thank you, Jeff. I will. Thanks.