You can't fill it with anything if it's actually a currently functioning "way." Do you have a picture that's taken from a distance so we can see how it fits in with its surroundings?

Sure, here's a wider shot:

The trough is mostly hidden by that spirea, which is badly in need of pruning. You can't see it, but there is a downspout running parallel to the long side of the trough behind the spirea, which presumably sends water into the trough and down through that drainage pipe.

I am planning to weed and mulch that whole area from the window well to the fence. The trough isn't offensively hideous now that we mucked it out, but I thought it might look nicer if we could fill it with some crushed stone or something (keeping the actual drainage pipe clear, of course).

Provided the volume of water that passes through it is not huge (no rivers running there seasonally :-)) you could fill it with rock without altering its drainage potential. Rather than gravel, I'd go with a larger diameter river rock (rounded sides).

Thanks gardengal, that's what we were thinking - a larger, smooth type of rock. It's not really an area where anyone will likely be spending a lot of time, but it would be nice to have something other than a mucky hole to look at, and I think it would be easier to remove leaves or other detritus from the top of rocks rather than digging it out of the hole. That said, what we dug out yesterday seems to be nicely composted ;).

Well, as far as I can tell the drainage pipe is buried in the yard and runs all the way down our long slope, then ends at the bottom near the street and empties into the gutters. Presumably this is what it's supposed to be doing. We don't have any moisture issues near the house that I'm aware of, so I think all of that is working fine. As far as the mucky compost issue is concerned, I just thought it might be easier to remove leaves, etc., from the top of large rocks before they become mucky, but perhaps you're right that a cover would be a simpler solution. It's a pretty big trough, though (maybe 5'x2'), so it would need to be fairly lightweight to be movable!

Not gravel - ROCK! And a relatively large diameter rock, like 2-3" minimum, similar to what one would use for a dry stream bed. This will still allow the water to flow easily and is moderately easily to keep clean as well as preventing someone stumbling into the trough and injuring themseves. You will always have some maintenance issues with a concrete lined trough regardless of how it is handled and this is not at all the same sort of application as using a rock or gravel mulch.

Do you really think that someone who does this for a living (and I am in a pretty damp climate as well) is gong to suggest something that is not a feasible and relatively low maintenance solution?? Sheesh!

While technically I don't disagree with GG that adding ROCK to it will work and allow water to continue. It will slow the water and it won't prevent things from eventually sifting down toward the bottom of the trough and silting up the rocks. Cleaning it out then will be a royal pain. Given that when drainage apparati clog or stop working for unknown reasons, it can cause a lot of havoc. I would discourage doing anything that would diminish the water carrying capacity of the trough ... though I think it is not a well thought out solution.

One thing we don't seem to know: Is this trough the SOURCE of water that runs through the pipe? Is it the COLLECTION POINT? If it is you could convert it to a normal yard drain which would be smaller and more easily maintained. It would require grading the watershed area that feeds it (which could be extremely small) to the drain. Again, since we're working with something that is not well thought out, we need to know what it is supposed to be doing before making changes. If it is so badly thought out that it is not really serving a genuine purpose, it could be removed. But we don't know that yet. Where is all the water that's going to enter this thing going to come from and how will it get in there? This must be SHOWN and explained, not just explained.

I will add that this structure looks potentially dangerous to anyone who might not expect it.

Yardvaark, I don't know whether this trough is a source or collection point. My best guess is that it's a collection point, and that the source is the downspout that runs just behind that spirea - that's the only source of water I know of nearby. Here's a closer photo with the downspout circled in blue (you can just see it peeking out from the greenery), and one taken earlier this year from behind the spirea.

The house is built into a hill and although the area immediately surrounding the house is flat-ish, there is a definite downward grade from the back (where the trough is located) to the front (where the drain pipe in the trough goes). This system seems to be working just fine - there are no water issues in the house nor any excessively wet areas in the yard. I agree that the trough poses a tripping hazard, although it is bordered on one side by a chain link fence and the other side is surrounded by an area where no one would go unless they wanted to go look in the trough. I have no plans to remove that fence or put in a gate for that very reason.

The next time you get a heavy rain, go out with an umbrella and observe where the water is coming from. That should give you an idea of your options.

In my previous house we had a corner in the front yard that stayed wet and actually became a spring in wet weather. I put in a french drain that also had a tee that received water coming off of the roof. Once I put in the drain, I was able to bury most of it. The source of the spring became a flowerbed for water loving plants (oakleaf hydrangea). I was able to have a lawn over most of the rest of the drain.

Before I installed the drain, the entire area was a muddy mess for most of the year.

I would do the check during a heavy rain, too, just to see if it will offer any clues.

If rain drops at any point within 6' of the house, does it run toward the house or away from it? If away, I'm speculating that whoever created this got a brilliant idea that was totally unnecessary. If that turns out to be the case, I'd remove it because it is a hazard. The person most in danger of being tripped by it or falling into it is someone doing house or yard maintenance.