The recent flood of illegal aliens across the Rio Grand river is straining the resources of Homeland Security.
The Posse Comitatus Act constrains the use of the Federal military in enforcing civil laws.
The US Coast Guard is NOT considered part of the US military, and routinely enforces Federal law.
The US Coast Guard is primarily deployed in maritime environs, and has many smaller watercraft perfect for riverine patrol duty.
All of these factors make the deployment of the US Coast Guard along the Rio Grand river the logical choice for protecting our border against illegal aliens.