Separating backwater from a creek

The creek adjoining my parcel opens to a 0.2-acre shallow (1ft or 2ft deep, perhaps) backwater.
More than once, a thought came to me to separate that backwater from the creek and introduce fish there;
but I don't have any idea how do do that other than dumping boulders and then gravel and then earth.
I looked up an article on Mother Earth News about creating pools, and it suggests laying timbers across the creek and filling in between them to make a dam;
but that's not exactly my case as I don't want to interfere with the main flow.

Could anyone give me an idea how it is done? Recommend a book, perhaps? I have zero background.

Or, perhaps, I don't even need to fully separate it to introduce fish?

Attached is the map; white and red thin lines are 2-feet contours; thick red is the parcel boundary; thick red segment between the promontory and the larger island labeled "26.61" is the distance in feet.
In the NW corner (note the interesting shape of the red contour line,) there is a trickling spring that I think kept the puddle from freezing at -20°F in 2014-2015 (and the last winter was too mild to tell.)

Interestingly, aerial pictures from 2005 (rather blurry) shows the present backwater area covered with vegetation, and the land I think was washed away after a fire (I keep finding charred pieces of wood in the ground.)

Flow is from NE to SW.

Thank you!