Here are photos from waller Alan Ash along with his own information.

Here are a few photos of a public works dry stone bridge that I've just completed 1/4 mile up a rugged trail going up to the top Spencers Butte high above the Willamette Valley here in Western Oregon. The seasonal creeks that flow through it can have a forceful flow in our wet season. Western Oregon as well as the Pacific Northwest (or the Northwet!! as it is called) has in areas, times of rainfall exceeding 90 inches a year!! The bridge is 65 foot long, requirements 4 foot wide at the top , 5 1/2 foot wide at the bottom, gravel on top for the trail specifications but no gravel dumped in the "hearting" of the wall.. Many, many stones in the 400 -600 lb range, Most all of the lower course is in that range and many of the capstones. The faces that you see on the stones are their smallest dimension, so it's easy to be mislead about their actual size. It was built to deal with the large amounts of water coming of f the hill as well as to carry powered wheelbarrows, weighing up to a ton fully loaded, to carry gravel for maintenance of the trail. The 65 ton of stone was hauled up the trail by these tracked powered wheelbarrows
Alan Ash /Ash Stone Masonry www.thestonemason.com