Agree with liquidsquid, it is not possible to compact soupy clay.

If your creek bottom is sand and silt, then I doubt you can pump it dry very effectively.

Does your creek ever go dry in August, or in a drought year? I hate to say it, but you might do much better on the cost and quality of your dam if you wait for a dry creek.

I believe most dams constructed on running water create a bypass and then build half of the dam. One option would be to take the excavator and cut a trench in the stream bed near one bank to a level below the base of your core trench. You will also have to extend this trench downstream far enough that the spillpoint in the streambed is lower than the base of your core trench. (This will make sense once you start observing the water. Basically, the top of the creek bed 200' downstream will still act as a "dam" and keep your trench full of water until the top of the creek bed is lower than the bottom of your core trench for the entire length of your drainage trench.)

Build one half of your dam on the opposite side of the creek and install your outlet piping system. When that is completed, backfill a section of your drainage trench to protect the uncompleted side of your core trench and build a bypass from the outlet to the drainage trench. Build a berm to divert all of the creek flow through your outlet pipe and then finish the other half of your core trench.

Other considerations:

Have you observed this creek during heavy rain events? Typically a dam in a flowing creek is going to require an exceptional emergency spillway for the dam to survive flooding events. The spillway will have to be large enough so that no floodwaters run over the top of your earthen dam, AND the spillway needs to be constructed to survive very erosive, high velocity water flows.

Also, in many states the requirements to build a dam on a flowing creek are much more restrictive than just gathering surface water coming in from a small watershed. Make sure you are in compliance with all state and federal regulations.

Good luck on your new pond project!