Study of 31 oval, enclosed tidal basins of a variety of sizes in soft sediments shows approximate dynamic equilibrium, regardless of basin origin, between average width and maximum depth, scour and fill, fetch (width) and wave base to the maximum observed depth of 16.5 feet. On the humid eastern coast, basins are commonly wide, shallow and directly alluviated by rivers. Equation of straight line average where y = depth in feet, and x = width in miles, is y = 0.41x + 3.0. On the non-humid southwestern coast, basins are proportionately narrower; straight line average is y = x, or a ratio of 1:1. Here, most elongate water bodies have become segmented to oval form by spits, bars, tidal deltas, and washover fans. Alluviation commonly overcomes disposal in inner segments entere by rivers, filling them while leaving outer segments in equilibrium. Depths in tidal channels are not used. Water bodies for which data are incomplete, those with very irregular outlines, and unsegmented parts of coastal lagoons are excluded.

With two exceptions, maximum departure from average depth during the record period has been 2-3 feet, 30 per cent, essentially within the range of incidental scour and fill and secular sea-level change. A shallowing of most basins since early surveys by 0.5-1.5 feet is believed to show mainly silting due to man's activities. Heavier silting and segmentation are caused by ship-channel excavation across basins. Segmentation, artificial or natural, tends to restore equilibrium, following or being followed by bottom filling. Modification of form is most evident after great storms or major engineering works.

In spite of postulated eustatic sea-level rise during the present century, the tidal basins have maintained width-depth ratios, in some cases actual depths, during equilibrium conditions in the cartographic period. Some bays sank 10+ feet in recent centuries but now have equilibrium of form.

Sedimentation in the basins is briefly studied in relation to the laws here investigated, to basin origin, and to its effect on bottom-dwelling organisms, chiefly the oyster.

The washover fan, a minor topographic and sedimentary feature of the lagoon side of offshore bars, has not been described before in the geologic literature of Texas.