The tribs they're talking about, in particular, are waterways feeding the Chesapeake Bay. Six major basins comprising 60 percent of the state's land drain into the tributaries which, in turn, drain into the bay. Each tributary is quite different so, naturally, each has different effects on the bay's waters. The Rappahannock River basin, for example, is mostly agricultural whereas the James River basin, Virginia's largest watershed, is more urban.
Different pollutants come from urban and agricultural areas.
That's the crux of the reasoning for bay trib strategies. It began in 1983. Leaders from the bay states - Virginia, Maryland. Pennsylvania and the District of Columbia - hammered out an agreement with the federal government to jointly improve the condition of the Chesapeake Bay. Signatories also agreed to maintain this reduced nutrient level once achieved, i.e. beyond the year 2000.