It doesn't really look like the leaves of your birch are showing signs of iron chlorosis since there isn't the green veining that you usually see. I wonder if the tree has been too wet this year. I would suggest a soil test to see what you have and then you could apply the proper fertilizer if necessary. Depending on its location you may need to provide water if the soil becomes dry since your tree is still fairly newly planted and the river birch does like good moisture but if your soil is heavy clay and doesn't drain well it could be difficult to grow anything. Here is a link to some info about growing river birch. http://www.clemson.edu/extension/hgic/plants/landscape/trees/hgic1005.html . Here is a link to info about getting a soil test. http://soiltest.cfans.umn.edu/ . Here is a link to info about iron chlorosis so you can compare. http://www.extension.umn.edu/garden/diagnose/plant/deciduous/birch/leavesyellow.html