Look at a flashboard riser system. It could be adapted to your situation.

Here's a similar set-up. One opening is in the pond, the other opening is attached to the pipe exiting the pond.

Agri Drain

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[Linked Image from lh6.ggpht.com]
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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).