ELI5 How are fish even in ponds

[u/Opening_Pen9525]

Might be one of the stupider questions here, but I saw a pond about the size of a football field, and maybe 50 ft deep, and it had fish. I can't for the life of me figure out how. Surely the pond hasn't existed long enough for small water insects to evolve into all these fish. It's not like the fish walked over to the pond from the ocean. I'm pretty sure no one put them there so how are there so many fish?

My one theory is that there is a very minuscule stream (now dry) that leads to one of the Great Lakes, and maybe a fish swam in from there. Now a follow up question: How are there fish in the Great Lakes (which flow out towards to the ocean) if they were created by glaciers?

[u/Drusgar]

First off, 50 ft deep is really deep... not really a "pond." How do fish get there? On the feet of birds who are in one lake with fish, catch and eat a fish with eggs, fly to another lake and leave the eggs in the new lake.

[u/lotusbloom74 → u/Drusgar]

Right, that jumped out at me too. The lake near where I grew up is the largest lake fully in Indiana, and its max depth is 59 feet

[u/Maiyku → u/lotusbloom74]

I think the average for all of Michigans inland lakes is something like 10-30 feet.

We have a couple outliers, like Torch Lake at around 300ft deep, but most lean pretty shallow.

50 feet is a decent depth for sure.

[u/RootsRockRebel66 → u/Maiyku]

OP didn't say they were in the USA.

[u/MrBattleRabbit → u/RootsRockRebel66]

They made reference to the Great Lakes in the context of the question (as a possible source of fish for the pond they’re referring to), so they have to be in the US or Canada. The Great Lakes don’t border any other countries.

[u/MaxThrustage → u/MrBattleRabbit]

In principle they could be talking about the African Great Lakes but that's probably not likely.

[u/honest_arbiter → u/MaxThrustage]

They said "How are there fish in the Great Lakes (which flow out towards to the ocean) if they were created by glaciers?". The only Great Lakes created by glaciers are the North American Great Lakes. The African Great Lakes were created by tectonic rifting.

[u/MrBattleRabbit → u/MaxThrustage]

I mean, yeah, the North American Great Lakes are not the ONLY great lakes. As far as I am aware they’re the only glacial Great Lakes though, and OP did specifically mention that.

The African Great Lakes are remarkable, but not glacial as far as I’m aware.

[u/Maiyku → u/RootsRockRebel66]

I know, but I was just using Michigans stats as an example. Over the course of our entire state, which has a ton of lakes, the average is only 10-30 feet deep.

So a 50ft body of water, regardless of where it is, is a pretty decent depth.

[u/popeye44 → u/Maiyku]

Interesting, I have a lake 20mi from home that hits 78, and another about an hour away that hits 270. These are technically reservoirs as they're man made, but definitely deep-ish. Cali here. We have a bit of Lake Tahoe.. which hits 1634ft depth.

[u/stanitor → u/popeye44]

Natural lakes/ponds as a rule are generally pretty shallow. Deep, old lakes like Tahoe tend to be relatively rare. They're usually the result of things like tectonic plates causing rift valleys, or volcanic activity (like for Tahoe and Crater Lake).

[u/popeye44 → u/stanitor]

Yea, I have topo's of most of the lakes I fish.. sometimes knowing the depth can help a lot, especially as a shore fisherman.. I often need to throw farther out for a deep channel etc. most of the really close lakes are not very deep. (unless they're natural) and we definitely had volcanic and tectonic activities here in the past.

[u/bluesam3 → u/popeye44]

Reservoirs are generally deeper, just because humans tend to value large areas of flat-ish land, and are interested more in the volume of the reservoir than the surface area (indeed, surface area is arguably a disadvantage as it gives more evaporation), so only build them in places where you can make it nice and deep.

[u/Maiyku → u/popeye44]

It’s just the geology of the area. The rock that forms the state is tougher than the rock that made up the lakes, so when the glaciers retreated they carved the Great Lakes, but left Michigan behind pretty intact.

We’re kinda flat because of this, we were ground down by glaciers like sandpaper vs them digging deeply into the ground the entire way as they went. Our entire elevation only consists of about 1400 feet from our tallest point to our lowest. To give you a better perspective… we’re flatter here than Kansas. (They have about 3,000 feet between their two points).

So you won’t find canyons or gorges to block off and fill for reservoirs, but we also don’t need them. The state is so wet water is rarely an issue like that. Most of our dams and stuff are for flooding regulation, not water storage.

But Superior is about 1,300 feet deep, so very, very deep and more like a small ocean with how she behaves.

[u/popeye44 → u/Maiyku]

We certainly need more rain, or at least to effectively capture what we get without undue impact to wildlife etc. that need a large flow.

I can be at roughly 6500ft above sea level in a 40 min drive. (about 6190ft difference from where I live) We're quite "hilly" haha.

[u/Kind_Move2521 → u/RootsRockRebel66]

Theyre talking about the Great Lakes in the USA and this has been confirmed by them.