I'm not very familiar with the Powder Expedition and Dakota Territory but...

I assume you have read the reports in the Official Records for the period they were in Dakota Territory. If not, you can go to:

http://cdl.library.cornell.edu/moa/browse.monographs/waro.html

Select "simple search"

Enter "Sixth Michigan Cavalry" and click the "submit query" button.

Choose the last item listed which is "Series 1 - Volume 48 (Part II) 4 matches in 4 of 1484 pages".

The next page will show the four matches under "Search" about three fourths of the way down the page.