ah man just as i got 3 new migrants 1 of my dwarfs drowned...what the hell was he doing in the river? was he suicidal or what?
You remarked earlier that the water on your map had frozen, correct? Without any more information, I'll take a wild guess and say that the river was frozen when the migrants arrived. The drowned migrant was walking across the ice of the river when it thawed, dumping him in. Where he drowned.
I'm just guessing.
If that is the case, okay, good - but now you wonder how to stop it!
You have three options.
A) Restrict Traffic: Under the Designate Menu there is a Define Traffic areas options. There you can set tiles to favor(encourage) dwarves to walk over them, or discourage(disfavor). It does this by adding or subtracting values to the pathfinding equations. Using this tool, Restrict all the river tiles so that your dorfs won't walk on it when frozen(do the same for ponds and pools!). If cool stuff is on the other side they might have to get to later, (B)uild a Brid(g)e and use the Traffic Designation tool to un-restrict it.
http://df.magmawiki.com/index.php/TrafficB) Use your dorfs to dig out stairs or ramps along the entire length of the river, both sides. If a dwarf falls in, hopefully they'll swim to the side and crawl out of the river before they die. You can put the ramps(or stairs) every few tiles if you don't feel like doing it the whole way.
C) Use both A&B, which might be the safest way to handle it.
D) Build a massive semi-automated cistern system underground that is fully self powered and controlled by levers, pressure plates, drawbridges, and the heart of an orphaned child. Add plenty of catwalks, observation platforms, and breakrooms. Drain the river at the source by tunneling up into its bed. No more surface river=no more drowning.
I hope this helps!