Gabion weir/dam

I'm new to the forum and was hoping to get some advice or links to resources. I have a relatively flat Creek 20 ft wide and I'm wanting to build a gabion weir across it to build up enough head to run a small ram pump. The creek has a solid stone bottom and is normally 6-8 inches deep but floods occasionally. Is there any sort of rule of thumb or math i need to do for sizing the drain pipe. I've already checked local laws and there is no issue with building it as long as its under 4ft tall and i don't stop the flow of water off of the property or cause flooding upstream. Any other advice on construction would be appreciated. Thank you

Here are somethings dam builders are going to want to know:
  -what is the topology of your land along the stream (relatively flat could mean feet and those few feet difference on your property might mean alot for head);
  -the topology of the your land will also give insight into the natural dips and highs that could prove useful in retaining water;
  -what types of material do you have on your land (sand, clay, rock, percentages);
  -how fast is the flow of the stream?
  -finally, consider the ramifications of your dam breaking (how to slow down a small tidal wave).

Good luck!