No. Walnuts (Juglans) have compound leaves, with very short stems on each leaflet except the end (terminal) one; and even if your 'leaf' was a terminal leaflet, its shape is wrong. It also does not fit with other trees with walnut-like leaves, such as hickory (Carya).
I think it is a poplar, a species of Populus; quite possibly P. trichocarpa, black poplar.
If you go to this page and click the Previous and Next links repeatedly, you'll see various stages in the tree's folaige and flowers/fruit during the year.
Thank you Mike - I can see that now. The seed pods I found were on the ground and I couldn't see any in the tree itself. I think the other seeds had already exploded. We found lots of the fluffy seeds further on near some other trees.