Two new reports deliver sobering reminders of how development is slowly strangling our rivers and streams. 

Just out is the Potomac Conservancy's 2nd annual report on the state of the "nation's river," which finds the biggest threat coming from pollution washing off the increasingly hardened landscape of the watershed.

Water quality and fish start to suffer when as little as 10 percent of a watershed gets paved over, researchers have found. In some of the more intensely urbranized stretches of the Potomac - in and around the District of Columbia - 45 percent or more of the land is covered with pavement, roofs and other impervious surfaces that prevent rainfall from soaking into the ground, the conservancy's report notes.  The photo at right, from the Natural Resources Defense Council, gives an aerial view of where the Anacostia and Potomac rivers meet in Washington.

With nowhere else to go, the rain gets funneled by storm drains into streams, turning their gurgling flows into muddy torrents tearing away at streambanks.   Fish, and the bugs on which they feed have a hard time surviving in such conditions. 

While pavement is spreading, tree cover is diminishing, making the river and its tributaries even more inhospitable to fish, the report says.  Trees soak up rainfall and shade streams so fish don't get overheated on hot days.  But in the District, the tree canopy has declined by 16 percent over the past 30 years, while stormwater runoff has increased by 34 percent.

"We're gobbling up more land, and that's in large part why stormwater runoff is the fastest growing source of pollution in the Potomac waterhsed," conservancy president Hedrick Belin said when I reached him to talk about the report.

Nearly half of the Potomac watershed is in Maryland.  A 2002 assessment found that two-thirds of the river's tributaries in the state were impaired.

The other report comes via my colleague Candy Thomson, the Baltimore Sun's outdoors writer, who reported in her column on Sunday about a new Department of Natural Resources study showing that central Maryland is losing its trout streams, despite costly restoration efforts.  Trout are especially vulnerable to loss of tree cover, which warms the water, and to sediment pollution that can smother their eggs on stream bottoms

"Biologists compiled more than three decades of aerial photos and ground surveys to show that brook trout have lost their fin-hold in six streams in the Baltimore area," Candy wrote.  The troutless tally: Baisman Run in Cockeysville, Sawmill Branch near Phoenix, Stillwater Creek near Eldersburg, Timber Run near Reisterstown, Red Run in Owings Mills and Goodwin Run. 

(Goodwin Run, it should be noted, was the focus of a restoration project I wrote about 16 years ago.  At the time, DNR biologists said they thought it was unlikely to succeed longterm, with ongoing development in the headwaters.)