WHEN WE THINK of a public park it's the trees and grass that come to mind, but when we actually visit one, it's the benches and the fountains and the footbridges that we use.

A modest National Park Service exhibit now on view at Pierce Mill celebrates Rock Creek Park's centennial by reminding us of what it takes to build and maintain our "natural" rest and recreation areas.

The exhibit's setting could hardly be more appropriate. Pierce Mill, built in 1820 and the last of eight grist mills that used the creek's power to grind Washington's grain, was rescued and restored by the park service as one of the many amenities of the Federal City's great central greenway.

Much of the "infrastructure" that keeps Washington green goes unseen, such as the massive water gate that gave its name to the scandal and also keeps the Tidal Basin from living up to its name. If the basin surged and ebbed with the Potomac's flow, it would be foul with flotsam and would periodically expose reeking mud flats.

East Potomac Park (Hains Point) is entirely artificial, a huge landfill supported by vast mats of willow branches that keep the ground from slumping into the river muck. It's a trick we learned from the Romans; so long as it remains wet and covered, the willow will endure forever -- or close enough to forever for human purposes.

An unspoken element of the exhibit is the sad state into which much of what makes Washington livable has fallen. Hains Point was for generations a haven where young lovers came to park and "watch the submarine races." Later they brought their children there to fish and play and picnic, or for a snack and soda pop at the 1920s Girl Scout Tea House (long closed and in ruinous condition, it was demolished in 1987). Groups of neighbors brought blankets and slept out in the cool river breezes that relieved muggy summer nights.

But in recent years Hains Point has been pretty much taken over by thugs and drugs, and now police close the road in midafternoon on weekends.. Meridian Hill Park once was a glorious mid-city oasis; now you stroll there at your own risk.

And so it goes. But the exhibit's a gentle reminder of the pleasant places that remain, and Pierce Mill, which is in full working condition, is itself worth the trip.