SMALL RIVER OUTLETS EXPOSED TO LITTORAL DRIFT Rivers with a small discharge flowing into the sea on an exposed coast are more or less obstructed at their outlet by drift of shingle or sand along the coast. When the flow falls 'very low in dry weather, the outlet of a river is sometimes completely closed by a continuous line of beach, any inland or tidal waters merely trickling through the obstruction ; and it is only on the descent of floods that the outlet is opened out. In rivers which always have a fair fresh-water discharge, or a small fresh-water flow combined with a tidal flow and ebb, the channel sometimes has its direct outlet closed, and is deflected parallel to the shore till it reaches a weak place in the line of beach, through which a new outlet is formed; or, where the current keeps the outlet open, a bar is formed across the entrance by the littoral drift, reducing the navigable depth. (See HARBOURS and JETTY.) Jetties at River Outlets.—The bar formed across the outlet of a river not heavily charged with sediment and flowing into a tideless sea can be lowered by carrying out solid jetties on each side of the outlet across the foreshore, so as to scour the bar by concentrating the issuing current over it. Thus by means of jetties slightly curved in plan, aided by dredging, the depth at the entrance to the Swine mouth of the Oder has been increased from 7 ft. to 27 ft.; the approach channels to the river Pernau and other rivers flowing into the Baltic have been deepened by jetties, and the outlet channels of some of the rivers flowing into the Great Lakes of North America have been improved by per manent jetties and dredging.
Where the littoral drift is powerful enough to divert the outlet of a river, as in the case of the river Yare, which at one time was driven to an outlet 4 m. south of its direct course into the sea at Yarmouth, and the river Adour in France, whose outlet, owing to the violent storms of the Bay of Biscay, was liable to be shifted 18 m. from its proper position, it has proved practicable to fix as well as to deepen the outlet by means of jetties. In such cases, however, where the rivers flow into tidal seas, it is impor tant to place the jetties sufficiently apart to avoid any loss of tidal influx, since the tidal flow assists the fresh-water discharge in keeping the outlet open ; whereas, with rivers flowing into tideless seas, a moderate restriction of the width between the jetties increases the scour. The tortuous and somewhat shifting outlet channel of the Scheur branch of the river Maas emerging on to a sandy coast where the rise of tide is small, and obstructed at its mouth by a bar, has been replaced by a straight cut across the Hook of Holland. The outlet across the foreshore is fixed in position by fascine-mattress jetties (see JETTY), the maintenance of the depth at the mouth by the tidal and fresh waters being aided by frequent dredging (fig. 3).