looks a lot like Weeping Birch (Betula pendula).

HTH
Chris

Here is a link that might be useful: Betula

This is Betula pendula (European White Birch). Wonderful tree (childhood favorite). All the white barked birches are very susceptible to Bronze Birch Borer which are pretty much impossible to control once there is an infestation and the result is needing to remove the tree. In order to prevent them, the tree needs to be kept as healthy as possible. They are river bed trees that have high water requirements in hot weather, and prefer full sun. Hope this helps!

I would not be quite so certain that it is the plain species of Betula pendula. I think it is a bit too white, unless someone has been peeling the bark to keep it so. There are other birches with white bark, including B pendula cultivars.

If black "crayon markings" on trunk visible in photo become more numerous toward ground, nearly covering base of trunk then that fits typical B. pendula. Whiteness varies with climate and exposure, as well as genetics. For instance, native variety of paper birch found out here not as white as those from eastern North America.

I bit my lip as well over whether or not it was (positively) a B. pendula or another species or cultivar. I decided to provide the poster with the name of the one with a weeping habit that he seemed most interested in. I also considered that the trees in the photos don't look to be quite mature, not a very full canopy at any rate (even considering the lack of foliage).
Just a side note regarding the black markings on the trunk...While some amount of black on the trunks is typical, when a lot is present, it is usually indicative of European Birch Aphid, which excrete honeydew, that in turn collects black sooty mold. (Perhaps what last response was referring to as being typical of the species)

I was talking about the black diamond patterns and fissures in the bark, characteristic of this species, and not sooty mold, which forms a dirty patina ("soot") all over infested trees (beneath the insects generating the honeydew that the mold grows on).

I was not trying to discredit what you were saying. I apologize. I intended my information to elaborate on your comment some. When aphids are present on birch, the honeydew and therefore "soot" becomes concentrated in the areas of the fissures due to the rough texture there.

I was just thinking that B pendula growing in Calfornia probably would be whiter-looking than the ones I am familiar with. In my neck of the woods the trunks will be damp quite a lot of the time and could well look darker, especially with associated lichens, moulds etc. Apart from the whiteness the rest of the characteristics fit B pendula.