Building a new bridge across the Hudson could have disastrous consequences - for the river, for the Hudson Valley and for our city.

For most of us, crossing the Tappan Zee Bridge is routine. For half a century, it's been the way upstate from Riverdale, just another part of the route up the Thruway on business or to the discount stores at Woodbury or a second home in the Hudson Valley.

But the Tappan Zee is not just another road: think of it as a gigantic bulldozer, gouging and gobbling and reshaping the earth for hundreds of square miles.

Its presence has transformed Rockland and Putnam counties from farm country to suburb. As a result, the air we breathe is tainted by the fumes of commuters' cars; the watershed on which the city depends for 10 percent of its drinking water has been fouled; and the food we eat comes from farther away.

So the decision state highway engineers are due to make soon about whether to expand the capacity of the Tappan Zee Bridge will have major consequences on both sides of the span.

The Department of Transportation appears to be leaning toward building a new, wider bridge. Proponents say that one of a variety of mass transit options will curb the well-known tendency of new road capacity to attract ever-higher volumes of traffic.

The environmental organization Riverkeeper has sounded the alarm, contending that a wider bridge will spur the migration of city residents to rural communities. The construction of a new railroad tunnel due to be built 20 miles south of the bridge is a better mass transit alternative, it notes, offering those who live west of the Hudson a one-seat ride to Penn Station.

Alarmingly, the DOT has "streamlined" the process for reviewing its decisions and studying their consequences, raising the prospect that citizens will be cut out and reviews will be less than thorough.