Jixxxer, as Brian wisely stated, the pond level will be governed by the level of the discharge. Let's say that you bury the drainline two feet deep, put an elbow at the discharge end and install a riser that is cut off four inches above the bottom of the ditch. That will be the level that your pond water will always return to. The pond level may be adjusted by increasing or decreasing the height of the riser. There will be outflow any time the pond water level is higher than end of the riser and higher than the level of the water in the ditch. When the water level in the ditch is higher than the pond level and higher than the riser, there will be backflow unless you install a check valve.
Water can be drawn from as deep as you like if you install a 45 degree fitting with an additional length of pipe. If you choose to draw water off the bottom, a "T" can be put at the end of the intake and stabilized with a length of pipe coming off each side of the "T". A screen made of 1/4 inch or 1/2 inch hardware cloth can be placed over the intake(s). At the discharge I would use window screen secured with a long hose clamp for easy removal and cleaning.
Perhaps you should consider improving your ditch or consider building an eighteen to twenty four inch high levee around the pond, which would allow you to lengthen the riser. Of course, with a levee, the drain field would be lost.
Since the carrying capacity of a pipe is directly proportional to the area of the its inside dimension, a round pipe will be just fine. Besides, although they may exist, I've never seen fittings that would accommodate a flattened pipe.
