Flood, deluge, inundation, torrent, spate, cataract are comparable when they mean a great or overwhelming flow of or as if of water.
Flood basically implies the flowing of water, often in great abundance, over land not usually submerged; it therefore suggests usually something (as a stream) that exceeds or breaks its normal bounds, but it carries in itself no clear implication of the ultimate cause.
Deluge may apply to a flood that destroys or drowns but especially to a tremendous and continuous downpour of rain; the term seldom suggests a flood which results from a melting of snows, a rising river, or a tidal wave. In its extended uses it implies, sometimes hyperbolically, a power that, through force of numbers, of volume, or of quantity cannot be resisted or that sweeps one away.
Inundation implies a flood caused by a stream, lake, or exceptionally high tide overflowing adjacent land; it therefore stresses something that overspreads and extends far and wide.
Torrent implies an impetuous rushing or surging of waters (as of a river in flood or of a stream that follows a steep course). It stresses the violence and rapid movement of the stream rather than its destructiveness or its capacity for spreading far and wide, and in extended use may be applied to something that comes forth with the same suddenness, the same violence, and the same clear direction.
Spate refers literally to a stream that has suddenly become full, agitated, and turbulent under the influence of a spring freshet or violent rains; hence in its extended applications it suggests a sudden swelling or outpouring of what usually flows in a quiet stream.
Cataract denotes a waterfall or a steep rapids characterized by a great volume of water descending precipitously or headlong; it is sometimes applied to something (as a deluge of rain or of words) that suggests such a waterfall or rapids in its overwhelming downpour or rush.