tonight on "nightline" -- the big flood. tonight, the mississippi river is rising to record levels. expected to crest within hours. and residents in the flood zone scramble to save what they can. diane sawyer is where it's all happening in memphis as the flooding spreads. plus, chaz's story. the only child of sonny and cher. the little girl who grew up in the spotlight. now she's a he. and he documented every step of his transition. tonight, chaz bono tells us and shows us how and why he became a man. and cher's son. how does the legendary performer feel about losing a daughter and

gaining a son? >> i was really frightened in the beginning. >> chaz and cher on the impact of his radical decision on their relationship. good evening. i'm cynthia mcfadden. we begin with flooding on the mississippi. that mighty river is still rising tonight. it's expected to crest overnight in memphis, where president obama has declared a major disaster. as residents abandon homes. so why is the river running so high? 31 of the lower 48 states drain into the mississippi or its tributaries and many of those states saw record snowfalls this winter. it's a lot of water all at once. diane sawyer is in memphis for our report, mississippi rising. >> reporter: the swollen waters of the mississippi.

the highest the water has been since 1937 and up to 4 million residents vulnerable to its force. stand in it and you feel the pull, the rush. you can see the water coming up. you can see it rising as we speak. look at the speed. look at the speed of which it's moving across the road right now. and you can already see what it's done behind me to the restaurant, the bar there. heaven knows what those houses were over on the other side. best way to travel the flood zone is by boat. we got a tour on the amphibious alter rain vehicle, a kind of tank that transforms into a boat the minute it hit ace few feet of water. the county is using this vehicle to get around and rescue the stranded. people taken off their roofs. urging people to evacuate early. >> on our first mission out here, there was some hesitancy. on our second mission, there was no hesitancy. >> reporter: but for so many

neighborhoods, already too late. you see only rooftops. an intersection normally filled with cars. and the street sign, lynnmanor looking up with us, an eerie welcome. >> you're probably making -- your neighbors may not be flooded. >> reporter: here in memphis, the school moved classrooms to the second floor but the water has been lapping right up to their door and the kids told us they've been scared. does it make you scared at night when you go to sleep? >> yes. >> reporter: what do you do when you get scared at night? >> i get in bed with my parents. >> reporter: i want to say, i am proud of you for saying that on nationwide tv. in fact, these kids helped their parents use sandbags to build a three-foot wall in just three hours. still, they lost their gardens out back, their shed and their little amphitheater. i talked to this colonel, in charge of the army corps of

engineers, responsible for making sure these levees hold. what's the immensity of the force coming at them? >> as we're sitting here looking at the mississippi, in one second, we're seeing approximately 2 million cubic feet per second go through the mississippi right now. and if i took just one second of flow, that would fill a football field 40 feet, 44 feet high. >> reporter: he has 150 inspectors moving up and down the river, testing the levees 24 hours a day. you've got potentially, what, eight states affected before this is over? >> it is a huge, monumental flood. we're living in historic times at this point. >> reporter: one big concern, the water that seeps under the levee. you can see how it bubbles up the sand, undermining the foundation. but the levees have withstood seasonal flooding for 70 years. though they've never been tested like this before. and no one knows what will happen as the river stays at a

high peak. in the meantime, thousands people have already evacuated. >> the water's going to be with us quite some time. >> reporter: we went early to the command post where emergency responders were getting second by second updates. how off on it you think of katrina? new orleans and those levees draining under the mississippi? >> that's something most everybody's thinking about. when they see something in a national news event like that and then they start hearing and seeing these words, evacuate, they're wondering, are we next? >> reporter: late at night, we caught up with some of the 400 people staying in shelters. a lot of them at churches like this one, hope presbyterian. >> the last time we've been in the house, the water inside, it was about that deep inside. >> reporter: we sure wish you well. for those who try to remain in their home, even if the water doesn't come, other things might. there are electrical currents. there is sewage in the water.

and there's wildlife. one sighting of an alligator. also told the water moccasins are everywhere. they too are seeking safety, clinging to the trees. and of course anyone who comes back home is in danger from walking into a vat of snakes. and it won't be over for at least ten more days. ten more days of worry and wonder about the floods and about what's underneath. for "nightline," i'm diane sawyer, in memphis, tennessee. >> our thanks, diane. there will be continued coverage on "good morning america" and "world news" with diane sawyer tomorrow. just ahead, he grew up as a little girl, the child of musical legends. but said he always felt like a boy. now, chaz bono is a man. his transition recorded on videotape. how does he feel now? and what about his mother? no pie today, ted? no, no, i just paid my car insurance bill -- ouch.

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what does it mean to be a transgender person? in the case of chaz bono, famously born chasety, daughter of sonny and cher, it meant feel like a boy trapped in a girl's body. mentally male, physically female.

imagine, he says, waking up feeling just as you do right now but in the body of the opposite gender. what would you do? 40 years in the making, chaz bono made the most radical change of all. i spoke with him for tonight's "nightline" interview. >> and here's our own very special guest. >> tonight and every night. >> our little girl, chastity. say tonight. now's the time. >> good night, everybody, god bless you. >> this is the person the world remembers, a little blonde girl in her famous parent's arms. ♪ babe >> i can't tell you how many people throughout my life have said to me or wrote to me or whatever when i was a kid, i wished i was you. >> did we know what we were saying? >> no, i don't think so. i would say very little up top. >> this is chaz as he's always seen himself, unquestionably,

undeniably, male. you have changed a lot in the last year and a half since i saw you last. >> i know. >> your voice is much deeper. >> it is. >> there's a real beard growing. >> yeah. yep. this was how i was born. >> but feeling male and becoming male are two different things. >> how's that look? >> that looks great, man, thank you. >> you are all set, my man. >> if i was an average joe, i would have done this years ago. if i felt a tremendous responsibility not just for myself but what am i doing to my family? what am i going to put them through? >> chaz had his first sex change operation a year and a half ago. it took him nearly four decades to start the process known as transitioning. changing physically and legally from female to male. so are you a straight man now? >> yeah, i am. always was. i just didn't know it. >> it is a process that included hormone shots. and having his breasts removed, which document filmmakers captured.

let's talk about the top surgery. >> okay. >> as it's called and the decision to go to that step. because i think for lots of people, taking the hormones is one thing. >> if you're a man and you have breasts, any man would want to have them removed. it's scary for women to think about it because it's something they're really attached to. being a male and having breasts was about the worst thing that i could imagine. >> you're flat. you're flat. >> that's jennifer, chatz's fiance. when they started dating, she had no idea that her girlfriend would become a boyfriend. >> bye. >> in the beginning, i was terrified primarily of what other people would say and think. >> what was the worst thing you thought people would say? >> oh, my god, i thought people were going to throw dog feces at our door. >> how you feeling? >> pretty good. >> and when the bandages came off after top surgery, jennifer says she had quite a realization. >> i saw the reveal.

i had a relief go through me. i was like, oh. so this is how you're supposed to look. okay, now it all makes sense. >> but does it? >> love you. >> i love you too, babe. >> we asked chaz to take us on a trip down memory lane to help us understand the important stops in his journey from chastity to chaz. we drive by his elementary school. >> very happy memories. i was one of the guys at that school, definitely. >> the girl girl who hung out with the boys even dressed like them. and then -- >> seventh and eighth grade was a particularly hellacious year for me. puberty. i was at a school that just really wasn't appropriate for my needs. it was straight strict being an d academic school. >> uniforms? >> yeah. >> cute little skirts? >> exactly. >> sonny and cher divorced when chaz was 4. he writes in "transition, the story of how i became a man,"

that he could dress and act like a boy at his father's house. his father even called him fred. but at his mother's house, it wasn't so easy. you write in the book in a really painful way the things she would say. >> i grew up in the '70s and '80s. we didn't know anything about this. my mom has her first child who's a girl and had the kind of expectation that any mother would have of their first born daughter. >> she said things like, you look like a truck driver. you look like a slob. >> right. you know, i mean, i just dealt with it. >> when do you first feel, okay, i got it, i'm a lesbian? >> it was my 13th birthday actually. we were living in new york. my mom was doing a play there. i mistook, you know, sexual orientation for gender identity. i went for the obvious answer. >> chaz came out as a lesbian and dated women. there were serious long-term relationships. but something still wasn't quite right he says. >> i just thought that something

was wrong with me, like i'm not being a good lesbian i felt like. >> how were you failing as a lesbian? >> well, because i was really a man. never wanted to be a woman. that's how i was failing as a lesbi lesbian. >> in 2005, chaz met jennifer. soon, chaz decided to go through with gender reassignment and began taking testosterone, causing tensions to flare. >> jennifer, shut your mouth. what are you talking about? it looks fine. >> stop yelling at me. >> it looks fine, leave it. >> don't yell at me. >> the hormones change people. think he's leveled out right now. there was a period where i felt the testosterone was really match cheese mow and it was hard to deal with. >> the emotional changes have been for you it sounds like the ones that have been tricky. >> at that point when i was watching the female essence leave his body, was hard. >> i'm the same person. i mean, the essence of me is me.

i'm just a male version of me. >> well, she says it's now like dating your twin. >> right. i'm the male version of my former self. >> not as sweet. >> i disagree. this is her rap, you know, that's not me. i'm a sweetie pie. i'm a nice guy. >> well, she started out with a nice girl. >> yeah, but i have testosterone curio curiousing through my veins now. >> you say much more sexually aggressive? >> not aggressive sexually. just a higher sex drive. like all men. it's different. >> it's more personal and more frequent. >> anatomically, chaz remains a woman from the waist down. medically, it's easier to successfully remove breasts than create a penis. so can you feel like a man without a penis?

>> absolutely. i felt like a man before i was on testosterone. this is where i'm a man, always. that's not to say -- i'd like a penis. i really would like one. and i hope that some day i'll get one. >> and what is his famous mother's reaction? there's so much whispering and so much "cher doesn't really approve and cher doesn't really go for this." host: could switching to geico really save you 15% or more on car insurance? host: does the buck stop here?

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we return to the story of chaz bono. how does his famous mother feel about his decision to become a man? i asked him. and i asked her. ♪ show a little more ♪ show a little less cher has always known how to stop a show like in "burlesque."

what happened when she was faced with the shocking reality in her own life? how did she react when he daughter told her she wanted to become a man? >> she never said don't do this, or if you do this -- in fact, when we first talked about it, so supportive, it was like, you know, she was channeling gandhi or something. and then the reality hit her. >> cher says his mother freaked out after he began taking hormones when she heard his more ma mass u lynn voice. >> she needed the time to process it. >> we told chaz what cher said about his transition. >> i was really frightened in the beginning. and also i felt that the doctors did not do their due diligence. but she's still the same person. i mean, really, it's no difference. her hair's doctor. >> she has facial hair. he's had his breasts removed.

the pronouns are hard. >> it made him happy. >> you think he knows you support him now? >> yeah, i think so. >> so you no longer have a daughter? >> i have two sons. never thought that would be but, you know, you go through life and you get what you get. you get what belongs to you. >> i got to a point where i was willing to risk everything and every relationship because i finally -- meant for to me than anybody else. the fact she needed that time, did it hurt a little bit? a little bit. but it was really okay. >> so fill in the blank for me. the rift with your mother at this point is? >> the relationship with my mother is moving in the right direction. >> as is so much for him now. his book is in stores tomorrow. also tomorrow, chaz will appear live on "good morning america." and a final note, former california governor arnold schwarzenegger and his wife