Yellow Japanese iris does not sound right to me. Yellow Pseudacorus sounds more probable. Pseudacorus reside happily in standing water or beside it. Considered invasive. Run down the gal who donated them. Maybe she has a picture if she doesn't know the name. Or print a Pseudacorus picture and see if she recognizes it.

The other commonly yellow beardless are Spurias. Harder to mistake for a Japanese.

Not much help until their identity is verified. Japanese do not like their crowns underwater during winter. Pictures of them rising above water is staged to enhance their beauty during bloom. Your low area sounds great as long as the crowns of Siberians or Japanese do not sit under water for any length of time.

Invasive, oh, no! I am
not in this club, so don’t know any of the members. Just happened to run
across their annual show on Sunday and they were selling things too.

Would photos of their leaves or roots help to ID them as
Japanese iris or these Pseudacorus?

Golly, don't know.

Here’s a couple of photos. When I pulled them out of the bucket, I realized they’d been in there a while. All the roots
were grown together into a solid mat. Until I am ready to plant them, I don’t
want to cut or tear them apart. I don’t know how helpful this will be without a
good look at the roots.

When I first started researching, when I thought they
were Japanese, I ran across a thread that told how to ID the various kinds of
iris: it stated,

“Is there a definite midrib and the leaves are less than
1" wide ? – Japanese”.

That’s what made me think they WERE Japanese as I
had been told. There is a definite midrib and the size is right. When I looked
up Pseudacorus I could find no description of the leaves. Photos focused
on the flowers with no leaf close-ups either.

Hope it's clear enough. It was getting dark.

I found one yellow Japanese iris at San Marcos Growers. It looks more like a spuria! But a yellow does exist.

http://www.smgrowers.com/products/plants/plantdisplay.asp?plant_id=2782

I found pseudacorus foliage also has a midrib. I didn't see a width number.

I'd plant them and see what blooms. If not wanted one or two years won't create an impossible removal job. Or you could put them in 5 gal. cans. I bought a kid's swimming pool for potted Japanese many years ago. Nice clump there.

That's why I was excited to buy it! Thought I was getting a fabulous deal. I don't know enough about irises (apart from bearded) to be able to tell a spuria apart from the photos I've seen of pseudocorus or anything else. I've looked up a lot in the last few days.

When I was looking over the plants and zeroed in on the bucket 'o iris, 3 members came over. When I asked what kind, one said Japanese iris. I said, "Are those the ones that are sort of flat across the top?" She said, "Sort of... would you describe them that way? " and looked to the other 2 for agreement. One said, "yes, but not as flat on top as a Louisiana iris." I knew I'd seen all of those in photos, but never in person, so that was the best way I could think of to describe how they look versus a bearded iris.

That's how I would describe them. Maybe the one girl was thinking of the ones where the stardards are short and stand up with the styles in the middle?

When I was looking for yellows I noticed some sellers (new to me) were selling pseudacorus and pseudatas as ensatas (Japanese). Here's a link to a reliable site with accurate information and pictures.

http://www.ensata.com/HTML/PlantPages/Pseudatas.htm

Last week I googled “yellow japanese iris” to see what I could find. I found
several sites showing what looked to me like pseudacorus, identified as a Japanese
iris. Including one Japanese garden in Seattle!
And I don’t know enough to tell any of them apart.

One blog mentioned that he had a large collection of
Japanese, and showed a yellow one he ID’d as Japanese-Iris-Jin-You.
The dark markings on the petals look the same. Makes me wonder, if the one he
was growing is one of the crosses as described in the link you posted.

Beware of Google images and misidentification!! There are likely more incorrect ID's than there are correct ones :-) There are no pure yellow Japanese iris - they simply do not have the genetics to produce that color. Yellow iris identified as Japanese are most likely pseudatas, a cross between the yellow flag iris (I. pseudacorus) and a Japanese iris (I. ensata). btw, Iris 'Jin Yu' is considered a species iris - Iris maackii.

That’s about what I figured. Chances are
these people bought them mislabeled in the first place. To me most of these photos
of pseudacorus don’t look that much different from the pseudata crosses or the species one. I would need
to see side – by – side diagnostic photos of all (not to mention seeing some IN
PERSON) to be able to learn to ID what I was seeing.

Like when I was a scientific illustrator in
the mollusk department of a major museum: we had standard views we had to
render. Exact same top, bottom, and side views of everything. You could not
really be sure you could tell one species from another unless everything you
were looking at was drawn from the exact same angle. Only then could a scientist
make the minute distinctions sometimes needed to tell one species from another.