In the open ocean, the ebb and flow of the tide is barely noticeable, but in certain V-shaped bays or inlets, the tide enters the broad end and literally piles up as it moves up the bay. This buildup occurs in the Bay of Fundy, 77km/48mi wide at its mouth, narrowing and becoming shallower along its 233km/145mi length. Thus, the tide is squeezed as it travels the bay, a ripple increasing to a wave several feet high as it enters the rivers emptying into the bay. This wave is known as a "bore," a tidal wave of unusual height. At Moncton the bore varies from a few inches to nearly two feet. The highest bores occur when the earth, moon and sun are aligned. The tidal bore and changing levels of the Petitcodiac River are best viewed from the park. A small stream at low tide, the river lies in the centre of a bed of red mud. At high tide the river widens to 1.6km/1mi and the water level increases by up to 7m/23ft.