You can find the definition for instance in the Wikipedia article

         Impersonal Passive.

The valency of a transitive verb is usually two: it needs a subject and an object. The valency of an intransitive verb is usually one: It doesn´t need but a subject. The definition talks about this latter type. Impersonal passive changes an intransitive verb (whose valency is normally one) so that it doesn´t need either a subject or an object to be a grammatically acceptable sentence. Just like yıkanıldı in your example.

The meaning of an impersonal passive is different from the classical Latin passive. When impersonal is used it always means that some people (the speaker doesn´t know or care who) are performing the action. Always some people. Just like you described the meaning of your sentence.

I don´t claim Turkish passive is completely an impersonal but it has usage which can be defined impersonal.

I remember my professor always started his passive lectures with the same words: "Remember, in Finnish (sorry, again) we don´t actually have a passive, we have an impersonal." At this point I was fighting to keep my eyes open. Only after beginning to study Turkish I actually understood what the old man was saying.

Thank you, tunci, si++. I guess I have seen yapmak replacing another verb before like in

         yapabildiğiniz kadar bu hareketi tekrar edin

         yapabildiğiniz kadar az sözcük kullanın.

It is maybe notable that yapmak in the above examples is used in the modifying part of the sentence while the verb describing action is "saved" to be used as a predicate.

But it also seems that repeating the same word in Turkish is not such a style defect as in many other languages. I find a lot of examples like

         dizlerinizi kaldırabildiğiniz kadar yükseğe kaldırın

where the predicate verb stem is mentioned twice in the same sentence.

According to the definition ("decreases the valency of an intransitive verb to zero")

         (Nehirde) yıkanıldı

is an impersonal passive.

I recognize irony when I see it.

Of course, Henry, you are so clever. Just guess how many places I searched...

The English type auxiliaries are very handy when the speaker needs to repeat a verb:

         Please don’t take him just because you can (take him).

         She has to take the dog out every morning but I don’t (have to).

         I should buy new shoes but I can’t (buy new shoes).

         Everyone laughed, I didn’t (laugh).

         Forget if you can (forget).

         No one cares. I wish someone would (care).

I´m not going to try to translate them but do you see what I mean? If you try to say these things in Turkish is there any way of not repeating the same verb (in another form)?

In the timeless world of mythology Io´s crossing from one continent to another was followed by the crossing of the Argonauts *from a denia (?) to a sea. Jason, accompanied by such first superheroes like Heracles, Peleus, Telamon, Orpheus and Castor set off for a journey to the North, heading to the Black Sea in order to find the Golden Sheepskin which had been hidden in the country of Colchis.

* Annoyingly, there is one word here which can´t guess.

Taking the risk of talking to myself I still want to stress that what I just wrote was only an answer to si++´s question. Yes, in world languages reflexive and passive markings can live just fine in the same verb when we talk about impersonal passive. And passive voice does have impersonal use in Turkish. That´s all.

What comes to the original question about what these odd verb forms really consist of naturally I won´t even try to answer it. It is for the natives to take or leave.

This is a tough one. I am not sure I know. But it has to do with the impersonal usage of Turkish passive which is a special feature of it. Of course I would miss it completely is there wasn´t a similar thing in Finnish. That´s why I understand completely when scalpel says

         yıka|n|ıl|dı = ´wash´ + ´himself´ + ´is done´ + ´sometimes in the past´.

(Well, this is not what he said but this is how I understood it.)

In Finnish it would be

         pese|ydy|t|ti|in = ´wash´ + ´self´ + ´is done´ + ´sometimes in the past´ + poss sg 3rd,

which means some people (probably more than one) washed themselves in the past. This is our passive. It can be formed from intransive verbs as well as transitive. It is used for reflexives as well. When we use impersonal passive we always mean that some people did the action, we just don´t know who.

Geoffrey Lewis writes: "The most remarkable feature of the Turkish passive is its impersonal use: niçin yalan söyle-n-ir? ´why are lies told?, i.e. ´why do people tell lies?´ In this example the passive verb appears to have a subject, but impersonal passives are also regularly formed from intransitive verbs and then have no conceivable grammatical subject..."

I also like the Wikipedia article about impersonal passive. There are some Turkish and German examples there.

I am in Kuşadası, a little bit restless. I will shortly come to Uludağ.