The silt and sand form banks and bars at the mouth, the water is too shallow in winter and the current is too strong in summer, and, further, the bed of the river is continually shifting.
Considerable deposits of mud, silt and sand are accumulating in many of the estuaries.
As might be expected under these conditions, it ranges from fine sand to silt which approaches clay in texture.
But while the wisdom of one age thus succeeded in restricting within bounds the tidal water of the river, it was left to the greater wisdom of a succeeding age to improve upon' this arrangement by admitting these muddy waters to lay a fresh coat of rich silt on the exhausted soils.
Spring-tides are preferred, and so great is the quantity of mud in these rivers that from to to 15 acres have been known to be covered with silt from I to 3 ft.
The silt deposited after warping is exceedingly rich and capable of carrying any species of crop. It may be admitted in so small a quantity as only to act as a manure to arable soil, or in such a large quantity as to form a new soil.
The low sandy shore of the Delta, slowly increasing by the annual deposit of silt by the river, is mostly a barren area of sand-hills and salty waste land.
Its limits east and west are determined bi the higher ground of the deserts, to which the silt-laden waters 0 the Nile in flood time cannot reach.
This silt consists largely o alumina (about 48%) and calcium carbonate (18%) with smalle quantities of silica, oxide of iron and carbon.
The shallowness of the lower stream, where the current is sluggish, is probably due to the great quantities of silt brought down by these floods.
The upper end of the gulf is filling up with the silt brought down from the Cordillera.
One of the most interesting facts in the history of the Gondwana series is the occurrence near the base (in the Talchir group) of large striated boulders in a fine mud or silt, the boulders in one place resting upon rock (of Vindhyan age) which is also striated.
In the broad river basins the inundations deposit annually a fresh top-dressing of silt, thus superseding the necessity of manures.
In Behar it has begun to replace indigo, and some success was achieved in Orissa, Assam and Madras; but jute is a very exhausting crop, and requires to be planted in lands fertilized with silt or else with manure.
Towards the San Juan outlet its depth decreases to 6 or 8 ft., owing to the vast accumulation of the silt washed down into the lake by its principal Costa Rican affluent, the Rio Frio.
Even at that period, however, the silt brought down by the rivers rendered access to the harbour difficult, and the historian Philistus excavated a canal to give free access to the sea.
If such a city was to be on the Egyptian coast, there was only one possible site, behind the screen of the Pharos island and removed from the silt thrown out by Nile mouths.
The silt and mud brought down by these rivers is rich in clay and organic matter, and sometimes when dry contains as much as I% of nitrogen.
Others that are exceedingly productive are the black calcareous loam of the prairies, the calcareous silt of the bluff belt along the eastern border of the Delta, and the brown loam of the tableland in the central part of the state.
On the Coastal Plain the soil is generally sandy, but in nearly all parts of this region more or less marl abounds; south of the Neuse river the soil is mostly a loose sand, north of it there is more loam on the uplands, and in the lowlands the soil is usually compact with clay, silt or peat; toward the western border of the region the sand becomes coarser and some gravel is mixed with it.
The artificial harbour was formed (1807-1832) between the mainland and the picturesque island of Ireland's Eye, and preceded Kingstown as the station for the mail-packets from Great Britain, but was found after its construction to be liable to silt, and is now chiefly used by fishing-boats and yachts.
Similar effects are produced along the boulder-clay cliffs of the Baltic. Where the force of the waves on the beach produces its full effect the coarser material gets worn down to gravel, sand and silt, the finest particles remaining long suspended in the water to be finally deposited as mud in quiet bays.
Most of the large Texas rivers have deposited great quantities of silt along their lower courses on the Coastal Plain, where the current is often sluggish and the banks are periodically overflowed.
The soils of western Washington are chiefly glacial, those of eastern Washington chiefly volcanic. In the low tidewater district of the Puget Sound Basin an exceptionally productive soil has been made by the mixture of river silt and sea sand.
The Tempisque enters the Pacific at the head of the Gulf of Nicoya, and tends to silt up that already shallow inlet (5-10 fathoms) with its alluvial deposits.
Nearly all the Gulf coast rivers, however, are obstructed by bars owing to the quantity of silt brought down from the sierras and the prevailing winds and currents on the coast.
A curious deposit of an impalpably fine and unstratified silt, known by the German name bess, lies on the older drift sheets near the larger river courses of the upper Mississippi basin.
The valley floor is covered with a flood plain of fine silt, having a southward slope of only half a foot to a mile.
The great river receives an abundant load of silt from its tributaries, and takes up ano lays down silt from its own bed and banks with every change of velocity.
The load of silt borne down stream by the river finally, after many halts on the way, reaches the waters of the Gulf, where the decrease of velocity, aided by the salinity of the sea water, causes the formation of a remarkable delta, leaving less aggraded areas as shallow lakes (Lake Pontchartrain on the east, and Grand Lake on the west of the river).
The water should be made to run completely off and the ditches should become dry before the influx of the next tide, otherwise the silt will not incrust and the tide not have the same effect.
Occasional breaches during floods cause the overloaded stream to spread in a great lake over the surrounding country, where the silt covers the ground in consequence.
Sections of the Missouri flood plain made by the United States geological survey show a great variety of material of varying coarseness, the stream bed being scoured at one place, and filled at another by currents and floods of varying swiftness, so that sometimes the deposits are of coarse gravel, sometimes of fine sand, or of fine silt, and it is probable that any section of such an alluvial plain would show deposits of a similar character.
Covering the higher parts of the south-western Palaeozoic area in most places are rolling hills of boulder clay or stony moraines; while the lower levels are plains gently sloping toward the nearest of the Great Lakes and sheeted with silt deposited in more ancient lakes when the St Lawrence outlet was blocked with ice at the end of the glacial period.