Are gnats and black flies the same thing?
The word "gnat" can refer to many kinds of small flies. But, if you live in western Maryland and are bothered by gnats that form large swarms around your face and ears, you are likely experiencing black flies.

I've been to Maine and experienced their black flies. Why are the black flies here so different?
The primary nuisance species in the Mid-Atlantic States, Simulium jenningsi, breeds continuously throughout the summer, but does not bite as often or as painfully as other species. In other parts of North America, local black fly species may breed only once per year. These species are extremely abundant, and painful, for a few weeks but are gone for the rest of the year. 

Where are these black flies coming from?
Black flies develop as larvae in fast flowing water such as streams. In the case of the Maryland black flies, our problem species Simulium jenningsi breeds only in large streams and rivers. Specifically, we have found these larvae in the Potomac and three of its larger tributaries (see our updated map here). While you may find black fly larvae in a small stream near your property, these are certainly one of the several non-nuisance species found in the region.

How far can the flies travel from their breeding source?
Black flies are surprisingly strong fliers. Female Simulium jenningsi can fly nearly 35 miles away from their larval source in search of blood (Amrine 1982).

References cited

Amrine, J. W., 1982. The New River connection to the black fly problem in southern West Virginia. WVU Agricultural and Forestry Experiment Station bulletin 678.