river in northern England. It rises in the fells (uplands) that connect the Lake District with the highlands of the Pennines and flows 90 miles (145 km) northwestward to its estuary in the Solway Firth, an Irish Sea inlet. From Kirkby Stephen, where its narrow, steep-sided upper valley opens out into the lowland vale, it flows in a meandering course among moraine hummocks (mounds of glacial debris) past Appleby, which is sited on a river peninsula. It receives short, swift right-bank tributaries from the great escarpment of the Pennines and longer left-bank tributaries from the Lake District and its flanking limestone hills. Its main tributary, the Eamont, entering near Penrith, collects drainage from the heart of the Lake District, including the discharge from Ullswater. Above Carlisle it receives the Irthing, which collects the drainage from the fells lying north and south of the Tyne gap near the Northumberland border. Carlisle commands the lowest bridge from a defensive site on the south bank, flanked by left-bank tributaries, the Petteril and Caldew. Even below Carlisle the Eden is not navigable. It discharges by shifting channels through tidal flats into Solway Firth.