Sorry you are all over the place with info that I cannot make sense of. What is the elevation change from the creek to the INLET of the sump pit? Does the sump pit service your exterior perimeter footing draintile or the inside perimeter?

I very roughly estimated 9 inches of fall from the bottom of the sump pit to where water emerges from the creek's bank. I don't know exactly what the sump pit is doing, which is why I'm asking the question, i.e. what was typical in that time period? There's no evidence of a retrofitted interior drainage system. There are two 4" pipes entering/exiting the pit. One goes to at least the basement floor drains. The other has constant outflow, ending up in the creek. A lot of inflow seems to be seepage; the pit itself has some gaps in the concrete. Would a direct connection to the creek even have been permitted by code at the time? Varies by jurisdiction?

Without speculating at the layout I would dye test the system. This will give you definitive results. Discharging groundwater and downspouts into the creek would be acceptable. The basement and garage drain may be subject to a connection to the sanitary. This ruling does indeed vary by local ordinance based on the utility of the creek. I recommend you get some help from a plumber to outline your system.

I used dye to confirm that the sump pit water is flowing to the creek. Further dye testing might confirm the unused gutter inlets join the system, but I don't see that it will tell me how the water is reaching the creek. More exotic methods come to mind, including renting a camera snake or ground-penetrating radar unit.

"every few years we get a major rain event and the creek rises high enough that I assume substantial water is pushed back into the sump pit and the basement too..."

Have you actually witnessed that happening?

"What would standard construction have been at the time?"

I'm not an expert, but I suspect that practice varies somewhat by region. Have you asked your neighbors how their drainage systems work?

"there are several 4" pipes emerging from the ground that were intended to be used for gutter drainage, but weren't hooked up in the end."

Are you sure they were never connected? Maybe a previous owner discovered that they overwhelmed the drain and perhaps backed up into the cellar.

"Would a direct connection to the creek even have been permitted by code at the time?"

I have no idea, but a carefully phrased call to city hall may be worthwhile. They might have some info on your area's code history. They might also be able to tell you something about the permits (if any!) for your system.

"has the water simply made its own path from pipe around the house down to the creek?"

The discharge you see into the creek almost has to be deliberate. I would be very surprised if the sump found its own way to the creek.

If your area has a "call before you dig" hotline, they may have a map of (documented) underground pipes. That has to be less expense and hassle than running a camera down the line. Still, if the system was never permitted, they may not know anything about it.

"'every few years we get a major rain event and the creek rises high enough that I assume substantial water is pushed back into the sump pit and the basement too...'

"Have you actually witnessed that happening?"

I've witnessed the creek water rising to well above where sump water is entering. Water does rise in the sump pit and begins to seep into the basement. I can't say what relative volume is coming from the creek, but given that it appears to be the path of least resistance I think it's nontrivial; drainage is otherwise good around the house.

"Are you sure [the gutter drains] were never connected? Maybe a previous owner discovered that they overwhelmed the drain and perhaps backed up into the cellar."

It is possible they tried it already and had problems. Given the debris entering such a system you'd think the intent had to be for it to see daylight, yet so far I've found no evidence of it.

Thanks for the other ideas. I see one can buy a useable 50-foot camera snake now for only $50, so I think exploration with one is actually pretty feasible.

You may be able to rent one at a tool rental place.

If the bottom of the sump is gravel it may be that they put a gravel or sand filled trench leading from the sump to the creek bank and covered it up. I agree with the above posts that discharging groundwater to the creek isn't a problem. If it was dumped on the ground it would go there anyway, and presumably it's clean. Many municipalities prohibit discharging it to the sanitary sewer anyway since they don't need the extra volume during rain events, and it doesn't need to be treated.

If you have troubles with it backing up during high water you can install a pump and run a pipe outside somewhere - not buried but on the ground surface so it can run off into the creek.

For less than the price of a one-time rental I purchased parts for a usable drain scope. I just need to put it together so I can see what's actually going on. I do now have two pumps installed in the sump pit, but they may not be able to keep up with the incoming water if there's substantial reverse flow from high water in the creek.

The camera reveals a larger chamber about 16 feet out from the sump pit, either underneath the garage, or more likely in front of it. This is after a left elbow and a right elbow and then a sharp downturn in the 4" pipe. I get stuck at this point even though there's no obvious obstacle. Posting a picture in case anybody knows the nature of this larger pipe or chamber with a smaller exit pipe at the far end.

It's hard to tell much from the fuzzy picture, but could it be a catch basin? Maybe there's one in the driveway but it's been covered over, either accidentally or deliberately.

This would always have been ten feet below ground level with no obvious access, so a catch basin seems less likely. That oval-shaped drain seems somewhat unusual--kind of hoping somebody has seen something like that.

It had been a couple years since I did the dye test so I tossed some more in to make sure there's still only the one drainage point to the creek, and that is the case.

The outlet (if that's what it is) in your snap doesn't look oval to me. To me, it looks more like the bottom of the chamber you found (whatever it is) is covered in sediment, going half way up the round outlet.

I wonder if what you see is some kind of underground sump to which all or some of those downspout drains were tied.

This is certainly an intriguing mystery!

I think you're right about the sediment, but I'm still not sure it's round. Here's a better picture.

New theory: Based on the photo below showing the approach to the right elbow in 4" pipe I think the above is still 4" pipe, partly filled with sediment, as you said, and also somewhat squashed. The narrowing explains why the 2+" ball with the camera stops there every time. I'm going to try to find a smaller ball to use.

Mystery solved! I carved a custom end cover out of styrofoam for the snake end and camera, only about 1.5" in diameter. This was small enough to make it farther through the squashed thin-wall pipe--a good reason to use Schedule 40 for hard-to-access places, like 10 feet under the driveway. So this time, after passing through two somewhat squashed pipe sections, I got within 10 to 15 feet of the creek, and the final pipe section is perforated. I could only get a few feet into it because it is compressed to less than 1.5". In the picture the white mark on top is where my snake sticks.

So now the question is how to best improve the situation? I can pretty easily dig up the perforated pipe, though replacing it will require uncompressing and rounding out the solid pipe end. It would be more of a hassle trying to replace the two squashed sections of solid pipe, as they are under the driveway, which at least isn't paved yet.