The bottom intake siphon drain is constructed of plastic pipe. It was designed for ponds that were built without any means of lowering the water and to supplement existing drains. It is usually 4 to 6 inches in diameter but larger sizes can be assembled on the same principles. Actually, any material can be used that will allow an airtight seal and access for filling and stopping the siphon when necessary.
A siphon is simply a tube completely full of water with the discharge end lower than the intake end. It is powered by the differential in air pressure and the weight of the water in the down hill portion of the tubing. Water will flow as long as the water level in the pond or lake is higher than the discharge end of the siphon. When the water level drops low enough to allow air into the intake end the siphon will stop (break) and water will drain from the line. The siphon should be constructed with a flow control device on the discharge end. This allows the adjustment of the discharge to match the inflow to stabilize the water level.
Automatic siphoning can be built into the assembly so it starts when water is high and only lowers the water to a predetermined level.
Water enters the siphon drain at the bottom of the pond (the intake of the siphon) to remove the least productive (lowest oxygen) water in the pond first.
The above image shows operation as a bottom intake drain. For this kind of operation, the pipe has to pass through the dam at the desired high water line. This takes the anoxic bottom water out first, leaving the more oxygenated water in the pond longer.
The diagram above shows the operation as a siphon to keep the water level down When the siphon is operating, the pipe is completely full of water. As water flows into the pond it will flow out the siphon. Care must be taken during assembly to be certain that an air tight seal is obtained at all joints. Leaky joints will allow air into the siphon and it will either not start, stop flowing or "break" as the water level drops.
The drain will operate as an automatic siphon if the threaded
cap in the "Tee" at the water line is left in place. 
When the water level rises above
the pipe. The siphon will start and will run until it draws the
water down to a predetermined level where there is a vent cut in
the underside of the pipe. A rubber sleeve is used to slide over
and seal the vent hole when this feature is not used.
The above sketch is of a drain assembly (Not to scale).
You can click HERE to see the
full size sketch. Dimensions of pipe diameter, length, and
fittings, will vary with individual installations. The
stack on the discharge should only be used where extreme
winter temperatures will not freeze it solid.
Otherwise, a
valve or other device should be used to regulate flow. The pipe
does not have to be buried except where it passes through the dam
breast.
In a new pond a section of pipe with antiseep collars
should be buried through the base of the dam. The end of the pipe
in the pond can have a threaded collar (male) with a threaded Tee
(female) on it. The threads allow the upright section to turn or
swivel when pushed down into the water. Attached to the Tee is a
riser (section of pipe) the length of the proposed water depth.
This section can be pushed over to lower the water or maintain it
at any depth. 
A sleeve
of larger pipe that extends above the water surface can then be
placed over the riser so water will flow from the bottom of the
sleeve into the riser.
A
variation of the above description uses a Tee that is glued in
place with a sectional verticle upright. The water is lowered by
removing sections of the upright.