Git glad to have you on board as you can see this place is a lot of fun. Lots of good advice.
The spillway pipe is used to protect the emergency spillway. If you have a good place for the water to run around your dam then it is not needed. This has as much to do with the change in elevation as the venation. IF THE WATER WILL FALL FROM THE POND DOWN FIFTEEN FEET TO THE CREEK IT WILL TAKE A LONG SPILLWAY WITH GOOD GRASS TO NOT BE A PROBLEM.
There has been a lot of good info already—I will only second some of it.
Use good thick plastic.
Install it properly. The hand packing is the key.