Hey Maggie
Now it’s coming back to me. It’s the network analyst toolset (instead of the geometric network tools…very confusing) that return an attribute table instead of a selection. The network analyst tools are designed for working with road networks, but also work for stream networks.
From here it seems like you have 2 options:
1. Use python or model builder to iteratively select each dam and run a the trace tool with one dam selected at a time.
2. Build a network dataset and use those tools.
From what I remember, setting up the other type of network dataset was significantly more complicated, but the tools are little more robust. Personally, I think I would lean towards option 1.