Yes, consider the brooks like a "shallow" water, it is passable but has a "wet ground" under it so if you mine it out from the side or channel the brook from above you'll get a river.
Because the brook by default is not deep at all there's no place for fish.
Rivers are much more dangerous.
About saves... When you continue a fortress/start a new one it says "region1" etc.
Go to your /Dwarf Fortress/Data/Save folder, you'll see a list of folders there with the names "region1", "region2" etc.
These are your "worlds" in these folders. If you want to get rid of a world, just remove a folder.
If you want to start adventuring in the world where you already have a fortress but you want to stay with the fort, find the world folder's name (as I said when you continue to play it is displayed like "region#"), copy it and rename it. I.e. you had a "region2" world, copy it in the same folder and name it "region3".
Now go straight into the game, load the "region3" game (it appears after the 2nd one, it should be it's copy) and press ESC->Abandon Fortress.
Voila, you're in the same world. As a bonus feature, you can find your fortress and check it as an adventurer (the easiest way to find the place of your fortress if you forgot where had you embarked is to start a new "fortress mode" and to select the Reclaim option, then you can find your fortress in the list and determine it's position on the world map).