Near the lake the valley was formerly swampy, and at high-water the lake overflowed into it.
Such tides as set towards the Himalaya broke against their farther buttresses, leaving an interesting ethnographical flotsam in the northern valleys; but they never overflowed the Himalayan barrier.
Most of the small closed basins, however, contain "playas," or alkali mud flats, that are overflowed when the tributary streams are supplied with storm water.
But between the Lake of Zurich and the Walensee the huge desolate alluvial plain grew ever in size, while great damage was done by the river, which overflowed its bed and the dykes built to protect the region near it.
Its present deserted and malarious state is probably owing to the silting up of the mouth of the Silarus, which has overflowed its bed, and converted the plain into unproductive marshy ground.