(I'm new here, so take my advice with a grain of salt)
If I was going to go about this, this is what I would do (other's ideas so far that I enjoyed are combined in to this):
1) Take a test of water quality. See what the temperature and pH of the water is. You may also want to test water hardness, etc.
2) You are going about this wrong. You need to change your way of thinking. You said you occasionally see life there. Well, what that means to me is that life occasionally passes by. Instead of introducing more life, what you want to try to do is create an environment that life wants to stay in. Make a mini-dam, like that one guy said, or dig a shallow pool in the stream. Plant some aquatic plants in it. Once the plants are established, introduce some daphnia or other native aquatic equivalent. That is assuming they don't already show up.
Over the course of a few years, you will have a small ecosystem. You'll find that at the very least a toad or two will have moved in. And you can probably introduce native species to your river pool with little issue.
As a side note, if you do decide to create a pool, think carefully about your entrance and exit. Try to create calm areas for the plants and microbey-things. For example, if you have a straight stream, and you dig a hole just on one side of it, you will create a whirlpool. I think your best bet might be to do something like this:
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This is all assuming that the pool is as deep as the stream, if you make it deeper, it will be calmer and have less eddies. Also plants will help calm it down, too.