40. Whenever it appears to the Government that injury to the public health or public convenience, or to any canal, or to any land for which irrigation from a canal is available, has arisen or may arise from the obstruction of any river, stream or natural drainage-course, the Government may, by notification published in the official Gazette prohibit, within limits to be defined in such notification, the formation of any such obstruction, or may, within such limits, order the removal or other modification of such obstruction. Thereupon so much of the said river, stream or natural drainage channel as is comprised within such limits shall be held to be a drainage work as defined in section 3.