why is it that most spillways are put on one side or the other of a dam
i am getting ready to have a new pond built and was thinking that the shortes rout for the spillway would be the best and cheaper
seeing how the creek will be about in the middle of the dam why not just put the spillway there so it flowes strait back down into the creek

My guess would be to help control erosion.

What I normally see is a water level device usually about mid-dam, this can be an elaborate deal with open and shut valves or just a big L-shaped pipe running from the surface to the bottom of the dam and out the other side (dumps right into the creek you described) while the spillways on the side are there for those gullywashers and once in a 100yr events. I have also noticed dams that have 1-2 or even three concrete culverts placed in the middle of the dam that can handle LOTS of water, but take precautions on the back side or erosion WILL be a problem.

Right before i got on this site i had a buddy with a spillway in the middle of the damn. We got 5 inches of rain in about 3 hrs one night, the next day he calls me and ask to use my bobcat. I was like whats up he said my damn washed out last night. So i headed over there. When i got there, litterally the whole lake was being held in by about 2' of clay and on the back of the dam was a washed straight down 15-20 dropoff with about a 20 foot round hole full of water. He says lets fill this hole in with your bobcat. I said we better get off this damn cause it could go at any minute. He hired the local dirt man (pond builder also) he filled it in and moved the spillway to the far side of the damn. I would say another inch or couple more hours of heavy draining his whole lake would have been gone.. It was wild how deep and cut out it was and what little wall of dirt was holding in the hole lake there was water just seeping through this thin wall into that pool it had created behind the dam.. Now its fixed and works perfect..

Bluegillerkiller
my plan as of right now (always subject to change) is to poor a concrete slab with wing walls and head wall across the top
then all the way down the back will be a full bed of riprap walked in with the crawler
the riprap will be wider then the concrete

i always try to have 50' of spillway lenth or more with a berm on the inside to keep the water from eating on the back toe. i try to make it level side to side and dropping only a inch or two downstream. i choose the most natural side to achieve this so that i dont have to disturb the grass to get it done. i control the elevation and downstream drop with the berm.

unless i have a natural rock spillway like the one in these 2 photos. the pond was about 40 yrs old when we rebuilt it and there was no sign of erosion on this rock so we left it as it was. not a good picture but you can visualize it with both pictures. first one is close up second is looking across pond at it. the cut in the first picture is to the bottom of the rock it was done for depth change and fishing purposes

ok this is starting to make sence to me now
were the dam runs into the hill side it is going to run the water off at a slower pace then strait down the back of the dam

yes even with your concrete erosion can start below it on the backside and eat its way back up eventually making a water fall out of the concrete. but your using riprap which may stay in place. you might think about putting some concrete on the rip rap to glue them together so to speak. you might think about a properly sized drain pipe if you have a lot of watershed. this will cure a lot of things but just my opinion

all good advice there Tim
Thank you