The organic material will over time dissapear. That means the depressions will still exist. Filling them with soil would do what you wish. That is what I have done.

That organic matter will not "disappear" over time but the soil food Web will digest it and what is not used as a plant food will be incorporated into your soil as humus, the residual organic matter you find in any good soil. Whether there would be enough organic matter mixed into the soil to raise those depressions back to the level before depends on how much of the OM is left as well as how much was there at the start. It is quicker to bring in soil and fill, but not less expensive.
If soil is brought in to fill depressions in soil, where grass is the ground cover, the proper method of leveling is to cut the sod and roll it back (or remove it), fill in to just above the previous level, and put the sod back on place.

There's no grass involved. This is in dense shade. Maybe I'll mix it up. I'll put some top soil in there along with organic matter. If it settles down, I'll just add more organic matter. I have an unlimited source of leaves. I have some rocks that I could put in there. I'm not sure if that's a good idea. If in the future, if I decide to dig in that area, it will make it harder.

Filling with the leaves will be least disruptive to the tree roots, but also least effective in raising the level of the soil. The organic matter will decompose, leaving only a small fraction of the original volume.

Filling with soil will raise the level, but might cause some disturbance to the tree roots, as they will suddenly be much deeper than they'd expected to be, and will get less air and more water in their soil as a result, which might also kill off soil microbes the trees are depending on. But if you're only talking about a few inches of fill, then there's not much of a problem.

Filling with rocks would not do that, but if you're going to dig in the future, rocks will make that harder, but digging is going to disturb the tree roots.

Do you really need to fill in these depressions? Depressions are normal in wooded areas.

I would first consider carefully if the depressions really need to be filled in. If so, then I would use them as a handy repository for organic debris disposal, with the understanding that it could take years to completely fill them in. Mixing in some topsoil with the debris would speed the process while greatly reducing any negative impact on the tree roots.

When I've done lawn maintenance, the method for filling in depressions in lawns is to add an inch or two of sandy soil a year to the depression. This allows the grass to adjust to the new soil level by itself, so no digging out the grass and replacing it. Tree roots would also adjust to a few inches of new soil at a time.

I normally use bourbon to fill in a depression.

Sorry, couldn't help it! LOL

Tox

I fill in all kinds of depressions in that manner our property is made up of impromptu compost piles.Recently I noticed one side of my old Blue Spruce was looking poorlyand as their is a depression/slope there I started a compost area for supplemental feeding and it already looks better.Did the same for a maple a couple of years back and it is lovely now.I am a complusive composter!
Peter

Darn it, tox you beat me to it, though I was going to prescribe beer, rather than bourbon. ;-)

I have dogs that dog holes, and I fill the holes with leaves and grass clippings as I pass over them in the fall, which is the only time I mow with a bagger on the mower (I want to compost the chipped leaves).

If I pass over a hole, I take the bag off and dump the clippings in it, stomping them down and mounding up a tight pile. Over the winter the pile sinks and fills in the hole, at least somewhat. Then my rockin' Iowa lawn grows over the area in the summer.

Now, I am not aiming for perfection, and maybe I am filling some of the same holes two years in a row, but it is nearly effortless and uses free material.

I am disappointed your landscapers did not fix the mess they created. I consider that part of the job.