Weir's, if tight enough to hold back water should help drop out sediment even if they are not tight enough to maintain even a wet spot after the rains subside. My take is that you want the weir to cause a backup situation that ultimately slows the linear velocity of the water stream down enough to allow debris, caught up in the current, to settle out in the temporary pool. If the weir works well, eventually it will fill up with dirt and such and the water stream will be able to maintain its velocity and carry the debris on down to the pond. Hence weir maintenance instead of pond maintenance. And, anytime you slow the water velocity down, it is not eroding the creek as fast. I would place the weirs were they can create the largest pools possible yet allow for future clean out access with a skid steer or the like.
I agree with snrub...cool project! I hope it does not involve a hand shovel too much.
