delivered the opinion of the Court.
The relators, petitioners here, filed their petition in the-Supreme Court of the District of Columbia for a writ of mandamus to compel' the Secretary of . War and the Chief of Engineers to authorize the construction of a wharf' in the Potomac Rivеr within the District of Columbia adjacent to their land on the Virginia shore, the . construction being forbidden by § 10 of the Act of March 3, 1899, c. 425, 30 Stat. 1121, 1151, 33 U.S.C., § 403, “ except on plans recommended by the Chief of •Engineers and authorized by the Secretary of War,” The judgment of the Suрreme Court denying the writ was affirmed by the District Court of Appeals.
Petitioners claim title through a- grant to their predecessors in interest of a plot of upland lying in the
*354
State of Virginia, which extended at the time of the grant •to the Potomac River. The upland has been .enlarged by the recession of the river toward the north and it is •the contention of the petitioners that the enlargement is due to accretion, with the result that their ownership has been extended beyond the shore line of the river, as it existed at the time of the grant, to the present high water-line, á claim which is put in issue by the answer. But it is conceded that the bed of the river below high water mark, where the proposed' wharf is to be built,- lies within the District of Columbia and that title to it and soverеignty over it were vested in the United States by cession from the State of Maryland of the area constituting the present District of Columbia. See Maryland Laws, 2 Kilty, Sess. of November 1791, c. 45;
Smoot Sand & Gravel Corp.
v.
Washington Airport,
Petitioners have entеred into a contract for the sale of their lands, conditioned upon securing permission to build the wharf, which is to be built and used by the purchaser in connection with a plant to be established on the upland for the storage of gasoline. It is stipulated on the record that the proposed .wharf, which is to be constructed in conformity to plans approved by the Chief of Engi
*355
neers, will not interfere with navigation. Petitioners assert a right as riparian owners to build and maintain it upon two grounds, first, that by. the common law rule as developed in the United States, the ownership of land bordering on a navigable river carries with it the right to build and maintain below high water mark a wharf or other structure, not an obstruction to, navigation (see
Shively
v.
Bowlby,
“ The citizens of each state respectively shall have full proрerty in the. shores, of Patowmack River adjoining their lands, with all emoluments and advantages thereunto belonging, and the privilege of making and carrying out wharfs and other improvements, so as not to obstruct or injure the navigation of the river; ...”
Tfiey insist that as thе proposed wharf will not interfere with navigation and as plans for its construction have been approved by the Chief of Engineers, it is the legal duty of the Secretary of War, under § 10 of the Rivers and Harbors Appropriation Act of March 3,1899, to grant the desired.permit. It is conceded by the government that the only basis for the Secretary’s refusal to authorize the construction of the wharf is that it would be inimical to the establishment of the proposed George Washington Memorial Parkway authorized by Act of Congress of May 29, 1930, c. 354, 46 Stat. 482.
By this legislation Congress appropriated $7,500,000 for the construction of a parkway a part of which is to extend along the Virginia shore of the Potomac River from Mount Vernon to a point above the Great Falls. *356 It authorized the National Capital Park and Planning Commission “ to occupy such land belonging to the United States as may be necessary for the development and protection ” of the Parkway. Construction of the Parkway was ¿uthorized as a part of the federal-aid highway program and was made conditional upon the contribution by Maryland or Virginia or others of one-half the cost of the required lands, other than those of the United States. But the Commission was empowered, in its discretion, to advance the full cost of the Parkway upon securing undertakings from these states, upon terms prescribed by the statute, to repay one-hálf of the cost to the federal government. A part of the Parkway, the Mount Vernon Memorial Highway, extending along the Virginia shore of the river from Mount Vernon to a' point within the District of Columbia, a short distance below the land of -the petitioners, has been completed.
Pending this suit, but before its trial, the Park and Planning Commission, by resolutions of September 24-26, 1931, declared that certain lands of the United States, described by metes and bounds, running along the high water line of 1863 on the Virginia side of the river, as established by United States Coast Survey, and extending to the center line of the channel of the river, arе necessary for the development and protection of the Parkway. By further resolution, the Commission declared that it took complete and exclusive possession of tjiese lands, which include the river ..bed where it is proposed tо build the wharf and the upland claimed by petitioners by accretion: It directed that copies of the resolutions be posted on each parcel, which was done before the hearing in this suit. A description of each was also sent tо the Attorney General for the purpose of having suits filed under the Act of April 27, 1912, c. 96, 37 Stat. 93, which authorizes suits by the Attorney General to quiet title to lands adversely held or claimed lying under and adjacent to the Potomac *357 River within the District of Columbia. The plаns of the Commission also contemplate the construction of a highway across petitioners’ upland as a means of access to the Parkway. '
It is apparent that petitioners are entitled to the relief prayed only if'several doubtful questions are resolved in their favor. They aré (1) whether a mandatory duty is imposed upon the Secretary of War by § 10 of the Rivers and Harbors Appropriation Act to authorize the construction of the proposed wharf if he is satisfied thаt it will not' interfere with navigation; (2) whether in fact petitioners have title, by accretion, to the upland adjacent to the river at the point where it is proposed to build the wharf, and thus have the status of riparian owners; (3) whether even as ripаrian owners of land lying within Virginia, petitioners, in the absence of a legislative grant either by Maryland before the cession or by the United States after it, have a common law right to build a wharf on the adjacent lands of the United States lying in the bed of the rivеr, see
Casey’s Lessee
v.
Inloes,
The Government contends that in view of the nature of these questions the case is not an appropriate one for mandamus, since ordinarily mandamus against a public officer will not lie unless thе right of the petitioner and the duty of the officer, performance of which is to be commanded, are both clear. See
McLennan
v.
Wilbur,
But we find it unnecessary, in the circumstances of this case, to say what effect should be-given to these objections alone, whether considered each separately or together. Although thе remedy by mandamus is at law, its allowance is controlled by equitable principles, see
Duncan Townsite Co.
v.
Lane,
The court, in its discretion, may refuse mandamus to compel the doing of an idle аct,
Turner
v.
Fisher, supra,
209;
Wilson
v.
Blake,
Affirmed.
Notes
“Sec. 10. That the creation of any obstruction not affirmatively authorized by Congress, to the navigable capacity of any of the waters of the United States is hereby prohibited; and it shall not be lawful to build or commence the building of any wharf, pier, dolphin, boom, weir,'breakwater, bulkhead, jetty, or other structures in any port, roadstead, haven, harbor, canal, navigable river, or other water *359 of the United States outside established harbor lines, or where no harbor lines have been established, except on plans recommended by the Chief of Engineers and authorized by the Secretary of War; and ■it shall not be lawful to excavate on fill, or in any manner to alter or modify the course, location, condition, or capacity of, any port, roadstead, haven; harbor, canal, lake, harbor of refuge, or inclosure within the limits of any breakwater, or of the channel of any navigable water of the United States, unless the work has been recommended by the Chief of Engineers and authorized by the Secretary of War prior to beginning the same.” •