I am playing vanilla Dwarf Fortress and although I have selected an area to have a river, I can't seem to find it. I am ~30 floors deep into the cavern now and there was only a couple tiny murky pools of water, but no river. How do I find it? Do I need to actually strike it with one of my dwarfs when mining? Do I just need to be on the same level as the river to be notified of its existence?
As far as I know, rivers are on 'ground' level, most of the time. So it has to be visible without digging. Are you sure you selected the right area while embarking?
EDIT: I think you used the search while embarking and were looking for regions with a river in it. These rivers are visible on the region map. If you moved the embark area in such a way, the river isn't covered anymore, then there is no river on the playfield.
Rivers are visible as blue lines on the world map and regional map when you are selecting your embark point. When you use the arrow keys to position the cursor on a river on the regional map, you will see a more detailed view of the river on the local map (left-hand side of screen) and the name of the river (right-hand side of the screen.) Brooks don't show up on the regional map, only the local map, but they will be named as well.
What may have happened when you selected your embark destination is that you chose a region with a river running through it, but you didn't move the 3x3 or 4x4 embark square on the local map. If a river, stream or brook doesn't run through one of the embark squares, you won't have water flowing through your embark area. It will basically be off-map after you embark. Use the
umkh keys to change the position your embark area on the local map before embarking.
Another possibility is that the river is there, but just not in the area you are looking at, especially if you've been building your fortress near the center of the map. You may have to use
k and the arrow/shift-arrow keys to move all the way to each edge of the map when viewing the surface z-level to find the river. Depending on where you positioned your embark area, you might only have a couple river tiles way over in one corner of your map, which may be easy to miss.