Temporarily, I'd pack bentonite around it. I use "enviro-plug" from a local well company.
Ultimately, what you need to do is install a couple baffles that will stop the water from running along the pipe. I would put one in about 1 foot from the pond bank, and the other half way through the dyke. You can use plastic sheets, sheet-metal, or even mud flaps, but it either needs to be tight to the pipe, or caulked with a heavy duty product. I know that this sounds like a lot of work, but it is the best way to stop water from undercutting drain pipes. Maybe someone has a construction diagram that they can lead you to.
