Sluices

I have a pond that is in three pools. the water flows from the top pool
to the middle pool through two sluices and to the lower pool through one
sluice. The water is then pumped to the upper pool.

The pools are made out of pool line, and the material used to create the
sluices has decomposed. Could some one recommend some material for the
sluices and where to obtain that materials.

PS We bought the house last year so I do not know who built the pool or
where they got their materials.