Map of USACE Divisions and Districts

The colors indicate the divisions.

USACE Pittsburgh District posted

Question: why does the U.S. Army Corps of Engineers divide districts the way it does instead of by state? 

Answer: water doesn't respect mankind's pithy imaginary lines.

State lines may make more sense at face value, but to maintain year-round river navigation and reduce flood risk, hydrologists have to look at watershed system as a whole. 

For instance, the Pittsburgh District's footprint covers where the Allegheny and Monongahela rivers meet to form the Ohio River. So why does the district manage multiple dams 100+ miles away in West Virginia? Because those dams' outflows feed to where the Monongahela River begins. It's the same reason the district's footprint extends past southwestern Pennsylvania into parts of northwestern Maryland, southern New York and eastern Ohio. 

Dividing districts by watershed also means hydrologists can make more precise water-management decisions in retaining and releasing water because they can account for the second- and third-order effects within the same area. 

The results are balanced water levels, measured flows, and safer communities.