The divide between the streams that flow into the Delaware River and Delaware Bay on the west and those that flow into the Hudson River, Newark Bay, New York Bay, Raritan Bay, Sandy Hook Bay, and Atlantic Ocean on the east is a very irregular line extending from the northwest corner of the state to Cape May. It is so near the Delaware River and Delaware Bay that only a little more than one third of the state is drained into these waters. The tributaries which drain this basin are, therefore, small. The eastern drainage area contains several rivers of considerable size. The Wallkill River flows northeast through the northwestern part of this state and through the southeast corner of New York into the Hudson River. The Hackensack River rises in New York and flows south into Newark Bay. The Passaic River, which has as tributaries Saddle River, Pompton River, Ramapo River, Wanaque River with Greenwood Lake, Pequannock River, and Rockaway River, follows a very tortuous course and flows into Newark Bay. Just north of the middle of the state is the Raritan River, which rises near the Delaware River and flows into Raritan Bay. All the rivers which flow into the Atlantic Ocean or into the bays which border the state are affected by the tidal flow, and the largest are navigable for at least some parts of their courses. In the case of the Delaware the tide extends to Trenton. Important cities which are located on tidal waters are Hoboken, Jersey City, Bayonne, Newark, Elizabeth, New Brunswick, Perth Amboy, Long Branch, Asbury Park, Atlantic City, Camden, and Trenton. Between the Watchung Mountains on the east and south and the Highlands on the northwest is a broad, flat basin, which is drained by the Passaic River. Before the Glacial Period the Passaic River did not flow from this basin through the mountains at Little Falls and Paterson, as it does now. This area was then drained by a stream which crossed the Watchung Mountains near Summit and Millburn and flowed southeast into the sea. During the Glacial Period most of this basin was covered by the ice sheet. As the ice receded, this gap in the Watchung Mountains was filled with glacial drift and there was formed a large lake, which the geologists call Lake Passaic and which was drained for a long time by a stream which flowed southwest through the hills near Liberty Corner into the Raritan River. As the ice sheet receded still farther north, the gaps through the mountains at Little Falls and Paterson were exposed. As these gaps are lower than the older outlets, the water of Lake Passaic escaped through them over the present course of the Passaic River to Newark Bay, and the lake disappeared. These changes in the drainage of the Passaic basin explain the presence of much swampy land in this section.