Perigean and Apogean Tides When the moon is nearest to the earth in its orbit (at perigee), its tide-producing power is greater than average, resulting in perigean tides. These are 15-20 per cent greater than average. When the moon is farthest from the earth (in apogee), the tides are
called apogean tides, which are about 15-20 per cent les~ than average. Coincidence of spring and perigean tides results in an abnormally great tidal range, while when neap and apogean tides coincide the range is abnormally small.
RIVER TIDES Tides are experienced in the lower parts of many of the great rivers. These are known as tidal rivers, where either the coastal area has recently subsided or the ocean level has risen causing the lower part of the river to be drowned. Such water bodies are, actually, extensions of the sea itself, or estuaries. River tides are distinguishable from ocean tides by one characteristic: the interval between a low tide and the next high tide is shorter than the interval between a high tide and the next low tide.
TIDAL BORES When a tidal wave meets a tidal river, or estuary, a tidal bore is formed: where the outgoing river currents are strong and the tidal river rather shallow and funnel-shaped, the rapidly rising high water advances upstream lik~ a high ‘vertical wall, known as tidal bore. Bores occur at river mouths that face the direction of tidal surge and where there is a large tidal range. Rivers like the Amazon, Hooghly, Colorado, Tsientang, Elbe, Yangtze are characterised by tidal bores.
TIDAL CURRENTS The tidal changes in ocean level result in stream-like movements of water in and out of bays and tidal rivers known as tidal currents. Unusually strong tidal currents result where bays connect with the open ocean by narrow inlets.