Needs a clearer close-up of the foliage to be sure, but looks more like J. cinerea. Could also be one of the related Asian walnuts, or the hybrids between these and J. cinerea.
As the leaves look to be pretty long, with fairly high numbers of leaflets it might be an Asian walnut. More than one species of these is pretty prevalent in the local landscape, sometimes of quite large size. Big butternuts are present here also. I remember at least one Asian walnut being present in the collections at Van Dusen, some years ago, up near the white(?) apartment(?) building that sits as though part of the park. This might give you something to compare to. Butternut is of the same type as the Asian walnuts but tending to produce shorter leaves with fewer leaflets on a more normally proportioned-looking tree, with less coarse and long branching relative to a longer, not so low-forking trunk (the trunk going up straight for some ways in your picture may perhaps point to butternut - but of course the total combination of characters needs to be considered). Older manuals with keys like those of Bailey or Rehder may help you figure this out. You need something like these works that treats both the butternut and the similar Asian species.
Thanks for the extra pics! Looks most like Japanese Walnut Juglans ailantifolia, but as Ron says, the Asian walnuts are a tricky group to sort out. If I remember rightly, some botanists even lump all the Asian large-leaved walnuts into a single species, though most accept 2 [J. ailantifolia, J. mandschurica] or 3 [J. ailantifolia, J. mandschurica, J. cathayensis] species of them. There's a paper with a key here which might help: http://www.dendrology.org/Documents/IDS_03_P107_P130_Juglans.pdf
Michael and Ron--thank you very much. Will enjoy my reading and head to VanDusen this PM to look at the Chinese walnut at the back of the garden. I recall it being similar but not branched. E.
