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.

Fish on!,
Noel