3. _Conservators may exclude the public._--Notwithstanding anything in the first section contained, it shall be lawful for the Conservators from time to time to exclude the public for a limited period from specified portions of the said river, for purposes connected with the navigation, or with any public work or uses, or for the preservation of public order. 4. _Right of navigation to include anchoring and mooring._--The right of navigation hereinbefore described shall be deemed to include a right to anchor, moor, or remain stationary for a reasonable time in the ordinary course of pleasure navigation, subject to such restrictions as the Conservators shall from time to time by by-laws determine; and it shall be the duty of the Conservators to make special regulations for the prevention of annoyance to any occupier of a riparian residence, by reason of the loitering or delay of any house-boat or steam-launch, and for the prevention of the pollution of the river by the sewage of any house-boat or steam-launch. Provided that nothing in this Act, or in any by-law made thereunder, shall be construed to deprive any riparian owner of any legal rights in the soil or bed of the river which he may now possess, or of any legal remedies which he may now possess for prevention of anchoring, mooring, loitering, or delay of any boat or other vessel, or to give any riparian owner any right as against the public, which he did not possess before the passing of this Act, to exclude any person from entering upon or navigating any backwater, creek, channel, bay, inlet, or other water, whether deemed to be part of the River Thames as in this Act defined or not. Provided also, that the powers given by this clause shall be in addition to, and not to be deemed to be in substitution for, any powers already possessed by the Conservators. 5. _Riparian owner to remove obstructions unless maintained for twenty years._--Any person obstructing the navigation hereinbefore described, by means of any weir, bridge, piles, dam, chain, barrier, or other impediment, shall be liable to be called upon by the Conservators to remove the same, and his refusal to do so shall be deemed to be a continuing offence within the meaning of this Act, and the obstruction itself shall be deemed to be a nuisance to the navigation unless the same, or substantially the same, has been maintained for the period of twenty years before the commencement of this Act.