In the following passage, some words have been deleted. Fill in the blanks with the help of the alternatives given. Select the most appropriate option for each blank.

Comprehension:
Waterfalls, sometimes called cataracts, arise from an abrupt steepening of a river channel that causes the flow of water to drop vertically, or nearly so. Waterfalls of small (1)______ and lesser steepness are called cascades; the (2)______ is often applied to a series of small falls (3)______ a river. Still gentler reaches of rivers that nonetheless (4)______ turbulent flow and white water in response to a local (5)______ in channel gradient are rapids.