Rectangular Pools in a Brook - at the meeting of small tributaries of Howard Brook, northern Northboro

We cross (west to east) into the valley of a brook. Looking down, we see a large mound.

Let me call attention to the rectangular pool of water on the left in the above picture.
Here we are looking back upstream:

Here we are looking downstream. Note the "4 in a row" feeling looking over towards that large rock pile. Also note the smaller rectangular pools of water, like a system of sluices.

Beyond the large pile, there are a couple of other piles, and another valley with a small tributary. Over there, there is a similarly placed mound but more decrepit than the one here.

We'll come back and look at some details later. 

A guy at work was talking about leaching acorns and about tanning skins as reasons to want a pool of water. I do not think this place is a mill but what are pools and sluices?