I think you could make that work Chris, but there are problems. I have a pretty good sized waterfall on one end of my 5000 gallon pond with a skippy filter behind it. The water drops into a skimmer on the opposite end then is pumped underground looping the pond to the other end where it flows into the skippy filter then overflows the filter into the top of the waterfall and back into the pond. The skimmer catches the leaves and if the pump in the skimmer goes off the water level stays contained at normal level because of the grade level of the skimmer From what I understand of your plans you intend to have two discrete waterfalls with 2 seperate pumps one waterfall dumping into the pond and one on the opposite end overflowing out of the pond into a gravel catch basin with a pump in a well at the bottom of the basin--if your grades levels and openings are correct (meaning the take away opening out of the pond into the pondless reservoir doesnt overwhelm your pondless pumps pumping capacity it will work) but keep in mind that if that pump should fail say from a power failure your pond will drain to that level unless you have away to close off that waterfall. I like the idea of this being your skimmer for leaves and floating waste but remember you may also loose fish or snails, or etc. down this wasterfall also, where they will immediatly die since they will end up on the dry gravel of the pondless reservoir. You could overflow from the pond thru say 2 or 3 two inch pvc pipes with screens on one end to save your fish and valves in order to isolate the pond in case of pump failure. This defeats your purpose of useing the basin as a leaf catchall though because of the screens and I gaurantee if the pump fails or power goes off it will happen at the worst time for you to shut the valves (2 in the morning or when your on vacation, etc.) Dont get me wrong, I'm brainstorming your idea as I type this, and I think it CAN be done. To do it I think you will have to figure your normal water level at a total level (you said 3 feet) so lets go with 3' as your total level- you will need to figure the grade of the total pond rim or collar to maintain that level EXCEPT for the section that will feed the pondless waterfall. 3' is the normal pool then you need to have a dropdead level in case something goes wrong with the pump and allows the pond to drain--this level I call the dropdead level because it is the level you have determined the pond could drain to without causing problems--say 6 or 7 inches (the usual skimmer box vertical opening) DON'T misunderstand me--you dont want this to happen but if something goes wrong you have already planned for it by say keeping planting shelfs or shallow fish lagoons deeper than this 6 or 7 inches so fish and plants or whatever wont be impacted by pump failure. 6 or 7 inches give you a little bit of water level cushon also because your pond will never stay at the exact 3 foot level you have planed for it may lose a couple inches fairly quick in hot weather and because of running the waterfall or gain a couple in a thunderstorm. If it drops a couple you still have 4 or 5 inches to feed the pondless waterfall without starving the pump. At the bottom of the waterfall sitting imbedded in the gravel you would need to have a basin that will hold a certain amount of water before it overflows out of this basin into the surrounding gravel and down into the pump reservoir. I think this basin is needed in order to catch any fish, etc that wash down the waterfall accidently and keep them alive until you can move them back into the pond. Now the problems as I see them; 1. you can save the fish this way and still allow free movement of leaves and debris to leave the pond into the pondless area but by its very nature this debris is gonna acumilate not in the gravel but in the fish basin--which will need cleaned out quite often, JUST like a normal skimmer would have to be. 2. The biggest problem I see is that you have a dropdead water level of 6 or 7 inches deep and you have to calculate how WIDE this waterfall opening has to be in order to feed your pump correctly ( width of opening by depth of opening to calculate total water volume to the pump ) You have some major calculations to nail down. First this width once you figure and build it into your system will never change BUT unless you keep some sort of automatic float valve system working in the pond to always supply water to the pond to maintain the EXACT water level your total water volume to the pump WILL go up and down with pond level which will alternatly either starve your pump or overwhelm your pump. Second after calculating this water volume TO the pump (lets say in a perfect world your pond level stays static so your total volume to the pump is always the same--I dont know lets just say youve designed it to send EXACTLY 3600 gph to the pump--now in a perfect world you go out and buy a pump that will pump EXACTLY 3600 gph back into your pond from the pondless reservoir at the level of head that you figured for--SHAZAAM its PERFECT--3600 gph falls out of the pond into the pondless reservoir the pump sucks it up and sends EXACTLY 3600gph back to the pond the thing looks great and your pump gets exactly what it needs and the pond stays exactly at the 3' level its PERFECT. Unfortunatly Chris this aint a PERFECT WORLD, My old 3600gph might have pumped that when it was new but just like they say the minute your born you start dying--same thing with your pump as the monthes go buy its rating will slow decrease as it ages--also I have to pull mine and clean it every so often of string algae and debris or the volume falls off. A little fall off hurts nobody and even a bigger fall off just slows down my waterfall (which is how I know to pull it and clean the pump screens) A big fall off on your design means your pump cant keep up with your designed water volume so your pump gets overwhelmed and the pond falls eventually to your dropdead level--not a big deal if youve designed for it but still a pain in the butt since the pondless pit only holds so much and the rest overflows and is lost out on the ground. I dont want to kill your dream--thats how innovation and the next big thing that everybody decides to try happens--people dreaming up a new pond design or anything thats worthwhile benefits everybody and I hope you can make it work ( I thought it could as I started writting this and still believe its possible) however keep in mind that this is a great hobbie that we have fun doing-- but if things get to be to big of a pain in the keister it quickly stops being fun and you may give up on it and nobody here wants that. Dont give up on your Idea just give it a lot of thought on paper before you stick that shovel in the ground, believe me its a lot easier to get your design perfected on paper than it is down in a hole with a shovel. Goodluck Tim.
