I am trying to use a 4" flexible corrugated plastic pipe to siphon from
my pond into a small pool and then to a stream bed. I have no problem
getting the flow going, but after a few days the flow rate goes down.
Then after about a week it stops altogether.

I did not need to use quite the capacity of the whole 4" line (but
wanted the high water pressure it provided to form a fountain on the
downside) so I added a cap in the pond at the top/supply side with many
3/8" holes. I was thinking that originally having the whole end open
was causing enough of a whirlpool effect that air was slowly building
up in the line and causing the siphon to eventually be lost, but the
change has not seemed to have any effect, besides reducing the flow
rate.

ANY ideas? It is not a big deal to restart the siphon now, but I want
the pipe to be buried/hidden in rocks and gravel and then it will be
MUCH harder to re-establish the siphon.

Thanks,

Bruce