Identify this large (swamp) fly?
Tabanus nigrovittatus as my initial guess. "Greenhead horse fly."



Source

This is probably the biggest give-away for the genus Tabanus (namely the features of the veins at the wing tip, denoted R4 and R5, versus other Diptera orders), still searching out a good identification key to solidify the ID. Furthermore, also known as the salt marsh greenhead, a swamp is a good candidate to find these (1).

So what's particular is entomologists have gone through and collected many specimens, and data about their wing venation, and based on the Comstock-Needham system, you can pin presence/absence of major veins, and branching of said veins to specific types on insects. In this case, R refers to the Radius or 3rd longitudinal vein, and the number refers part of the branching that reaches the margin of the wing. Pg 156 here helps us ID Tabanus vs other orders.

Problematic with identifying these spp. is you need to get a good look at the sternites on the under side as well, if you look at how they ID Tabanus sudeticus here, but the photos tend to match nicely with those of T. nigrovittatus you can widely find.