Their are two 16" diameter poplar trees about halfway up on the backside of on my 100'long dam. I know thats not a good thing but im wandering if cutting them down with the potential root size they already have on them would be worse than letting them grow. Any opinions ?

Know nothing about poplars, but: If the roots die/rot it can and will leave a void that generally weakens the dam.

One of our irrigation reservoirs has a problem with trees. The state dam engineer has us cut and kill them all. Pretty much anything woody has to go. So far, no problems with the dying roots. The bigger treed were aspens, 4 inches in diameter.

Poplars are fast growers. It will take a long time for the roots to decompose. It shouldn't cause leaks.

Mike Otto - premier pond builder on the planet says if under 4" - remove trees. Dave I advise pond sealing clients of mine the same...not unlike a hydrated cork on a good bottle of red wine, live roots help create a seal, but when decomposing will create a void which can lead to fissures/leaks. I always advise my pond sealing clients to leave trees over 4" alone as a result.

Agree TJ. Biggest problem of a big tree is that it keeps growing.

You guys may have just answered one of my questions. I have 4 willow trees just on the BACKSIDE of my 2 acre pond dam that I have let grow for 4 years not thinking they would hurt anything that far away from the actual water. The trees have gotton about 12 foot tall and the trunk about 12 inch all the way around. Should I cut those down? I assume there are roots at least attempting to reach the pond water.They are probably 30 foot from the actual pond. Are they "drinking" too much pond water or are they far enough away to do no harm? They are not on the dam at all. Just been thinking about them lately and wondering if I should cut them before they get much larger.

Flame, sound like you are describing the area behind my pond dam. For me, I don't think they are hurting much. I have a much more serious problem with willows growing all along the shoreline. Never ending battle to keep the cut down and thinned out. I try to do a cut and spray which works but not sure it works in our winter months due to they are dormant for the most part..

A general rule of thumb regarding trees and their roots is that the roots extend out as far as the limbs do. This may give you and idea of how far the roots have grown from the trunk.

Some exceptions might be...

Osage Orange - these trees tend to break and or lean out over the years under there own weight and typically have a smaller diameter root span than their limb span.

Willows near water - these trees do a lot of leaning as they mature due to the softer/wetter soils that they grow in. I have a common black willow below my pond dam that leans entirely one direction (35-40 feet and 18" diameter at the base) and I am sure that the root mass does not extend that far out from the trunk, nor does it only expand in the leaned direction.

Trees growing in inconsistent soils - anytime the soils are in proximity to rock shelves, caves, clay pan, or undesirable soils... the rule of thumbs is out the window.

You have to imagine the leaner, gnarly, and/or damaged tree to be in a more upright position and assess the root mass diameter from that.

My several trees below the dam would worry me more if the pond was the main source of water, but the water table is insanely close to the surface here and i feel like the trees do not have to "search" for water from the pond.

Flame, I doubt that this answers your question, but hopefully it helps you asses your situation.

My forage pond dried up about 10 or so years ago. It has a big willow(water sucker) on the dam. I noticed a small willow on the other side. When I tried to pull it up, I found that it was from a root from the big willow about 40 ft away. Those things suck a lot of water, and are extremely prolific. Every year I either pull or poison them.

Mature willow or cottonwood can drink 500g water daily. Do the math...can be very expensive.

The willows behind my dam are in an area where we redirected the rain water so there is a water source other than the pond. I could cut them down but it's not an easy place to get to because it is always wet and muddy. The willows around the shore line grow like really fast I am sure there is a lot of water drinking going on. Seems like every leaf that drops into the water will sprout up a new tree.

I walk my place every 3-4 months and pull every willow or cottonwood I find around the 7 ponds...if the tap root won't budge, glove of death with glyphosate. I especially monitor the backsides of all my dams and nuke anything even cedars.

Something I've found better than Glyphosate or Tordon is Milestone.
Some tree/shrubs learn to close off cells when sick, not taking chemical to root full strength. If you run into this, try the Milestone.

Thank you all for your input i really appreciate it, it gives me a lot to think about. The one tree that concerns me the most is in the center of the dam and it comes out of the ground about 6' below what would be the surface of the water and the water is 11' deep at that point. In theory the drip line of the limbs would put the roots 3 or 4 feet from waters edge at that depth. Im not to concerned as to how much water the tree consumes my water shed works well. Like someone said my worry was the roots rotting an creating a waterway through my dam. With that being said poplar trees grow BIG around here. What are the possible down sides of leaving it since its already as big as it is all ive really heard about is the root rotting thing. And again thank you all in advance, i dont get access to the internet that often.