Topography: Hamana-ko consists of drowned valleys formed by the postglacial sea-level changes. The valleys are incised deeply toward north through a Pleistocene terrace into a Paleozoic basement mountainland and are closed at the mouth by sand originating from deltaic deposits of the Oi River, which drifted from east to west by the coastal current along the coast of Enshu-nada. In the northern area of the bay, the outline of the rocky shore formed by Paleozoic sedimentary rocks clearly indicates a drowned valley origin. In the western and eastern areas of the bay, the Pleistocene formation occurs as terraces with flat upper depositional surface. In the southern area of the bay, a coastal lowland of alluvial deposits develops along the Pacific Coast. Thus, the coastline of the bay is characterized by much indented topography with rocky inlets in the northern area and a straight shoreline in west-to-east direction along the Enshu-nada.

Drowned valleys have been filled up with sand transported toward the north from the mouth of the bay. A sandbank of less than 4 m depth occupying the southern half of the bay impedes water circulation between the open sea and the enabayment; a moderately deep area of stagnant water with a maximum depth of 16.1 m occupies the northern half of the bay. To circulate sea water toward the inner part of the bay, three artificial channels (maximum 4 m deep) have been excavated. The small adjoining inlets corresponding to the heads of the drowned valleys still retain their narrow entrances, which open toward the main bay, and are deeper (13-16 m deep) than the main bay. The continental shelf along the Enshu-nada is wide and occupies a flat bottomed area -2 km wide and less than 15 m deep-along the coast. The topography of this area is shown in text-figure 1.

Sediments: Three major types of sediment can be distinguished: poorly sorted gravel sand, well-sorted sand, and moderately well-sorted black silt and clay. Gravel sands are found in the shallow coastal areas along the Enshu-nada, on the bottom of the channel at the mouth of the bay, and in areas along the rocky shore in the northern area. Well-sorted sand is distributed in the southern half of the bay and is supplied by tidal currents from the open sea. Black silt and clay found in the northern half of the bay are of suspension origin and are supplied mainly by terrestrial runoff. The well-sorted sand is rapidly filling the southern half of the bay, whereas black silt and clay are slowly filling the inner part of the bay.

Noteworthy is the distribution of carbonaceous black mud, which is limited to the northern area of the central part of the bay and the inner area of the southeastern inlet. These areas are characterized by stagnant water, deposition of plant remains, and production of hydrogen sulphide. On the continental shelf the sediments are separated by sorting into two types: well-sorted coarse to medium sand along the shore which is the sorting product of the present wave action; and moderately sorted fine sand distributed offshore deeper than about 10 m, i.e., below the wave base. The general textural properties of the bottom sediments based on grain size analysis and their distribution in relation to the directions of the surface currents are shown in text-figure 2.

Oceanography: On the continental shelf, a strong coastal current runs from east to west along the Enshu-nada Coast. Water circulation between this coastal water and bay water is mainly controlled by tides through the narrow mouth of the bay. The wide southern sandbank area of the bay less than 1 m deep also prevents water circulation between the open sea and the inner part of the bay. To facilitate movement of sea water, three channels about 4 m in depth have been enlarged by dredging. Thus, the water from open sea can reach the entrances of the inlets and form a large sinistrally circulating bay current. The velocity of surface water current into the bay is 1-2.5 m/s in the bay mouth; 0.2-0.8 m/s in the mouth of main bay; and 0.03 m/s in the inner bay area (Nonaka et al., 1973). Terrestrial water flowing into the bay is carried by 14 small streams, among which the largest river, the Miyakoda-gawa, carries 1 m3/s on the average and the other streams less than 0.2 m3/s of flux.

The chlorinity decreases toward the inner part of the bay due to the influence of terrestrial water. Thus, the bay water is Oligohaline near the river mouths, Mesohaline in the main part of the bay, and Polyhaline near the bay mouth. The proximal part of the continental shelf outside of the bay along the Enshu-nada is covered by coastal water.

Vertical changes of temperature and salinity are indiscernible in the southern shallow area, which suggests the influence of a strong inflow of sea water. Yet, on the contrary, the water temperature and salinity of the northern part of the bay are characterized by wide seasonal variations. In winter, the mixing of the bay water takes place through a downward movement of the cooled upper water mass of high density into the lower water mass of low density. The difference between surface and bottom water temperature is not significant. In summer, the surface layer has a high temperature and low density, whereas the bottom layer has a low temperature and high salinity. Thus, the water becomes stagnant and stratified into two layers considerably different in temperature and salinity. This summer stratification is a phenomenon that exerts a strong influence on the deeper life of the bay. The dissolved oxygen content of the bottom water dwindles to zero in depths greater than 8 m, where hydrogen sulphide is produced in summer.

Recently, due to the development of pisciculture, the water environment of the bay is rapidly changing by an increase in the relative amount of sea water. According to the calculations of Nonaka et al. (1973), the amount of water interchanged every year between the bay and the sea is increasing up to about 23.3% of the bay water, and the amount of fresh water received by the bay is 6% in a year.

Nutritive salts are carried by terrestrial water and accumulate in general in the bottom water layer. They tend to decrease from the mouth of the streams toward the mouth of the bay and are especially abundant in the northern part of the east bay.

The concentration of organic calcium carbonate, represented mainly by shell fragments, is low in the well-sorted sand of the southern bay and high in the silt of the northern bay, especially along the bay coast.

Zostera nana and Z. marina, the characteristic species among aquatic plants of this area, offer a refuge for aquatic animals, including ostracods. The former species is distributed in several patches along the coast of the bay, while the distribution of the latter is confined to the border area between the northern and the southern parts of the bay.