The Rock Creek and Potomac Parkway was not the first extension of Rock Creek Park. Nearly a decade before the parkway's authorization, moves were afoot to add the first of several tributary stream valleys to the park. The Piney Branch Parkway, extending along the eastern tributary of that name to the vicinity of 16th Street, was proposed in legislation prepared by the District commissioners and transmitted to Congress in November 1905. The resulting act, approved February 27, 1907, directed the commissioners to institute condemnation of land along the valley "not more than an average of four hundred feet in width" connecting 16th Street and Rock Creek Park. 
The land was acquired by October 1908, and the Rock Creek Park Board of Control designated it Biddle Parkway for Col. John Biddle, a former board secretary and District engineer commissioner.  The name did not adhere in common use, but the parkway grew in length and width under later legal authority. In the 1920s it was extended beyond 16th Street, and the road permitting traffic from adjoining Arkansas Avenue beneath the 16th Street bridge down the valley to Beach Drive was constructed. Land acquired on the north slope of the valley included the prehistoric Piney Branch Quarry, now listed in the National Register of Historic Places.
Klingle Valley, running west from Rock Creek along the north side of the National Zoological Park, was another early object of acquisition interest. Congressional legislation was pending in 1912 to add a parkway strip along the existing Klingle Road from Rock Creek Park up to Woodley Road. In the early 1920s the Office of Public Buildings and Grounds was still pursuing this objective and was seeking a further extension to connect the proposed Klingle Valley Parkway with the Normanstone Parkway, running northwest above Massachusetts Avenue from the Rock Creek and Potomac Parkway. The connection would enable a parkway detour around the west side of the zoo, wanted because the road along Rock Creek through the zoo was closed at night and whenever water rose too high at the two fords within the zoo ground.  The Klingle Valley and Normanstone parkways ultimately came into being, their land acquisition continuing into the 1950s, but the connection between them never materialized.
In addition to serving for access routes into Rock Creek Park, these and other tributary park extensions were wanted to help preserve the Rock Creek watershed. Acquisition of the desired lands was a foremost purpose of the National Capital Park Commission, established by an act of Congress approved June 6, 1924, "to preserve the flow of water in Rock Creek, to prevent pollution of Rock Creek and the Potomac and Anacostia Rivers, to preserve forests and natural scenery in and about Washington, and to provide for the comprehensive systematic, and continuous development of the park, parkway, and playground system of the National Capital."  The commission was composed of the chief of engineers of the U.S. Army, the engineer commissioner of the District of Columbia, the director of the National Park Service, the chief of the U.S. Forest Service, the officer in charge of Public Buildings and Grounds, and the chairmen of the House and Senate committees on the District of Columbia. The officer in charge of Public Buildings and Grounds (succeeded in 1925 by the director of the Office of Public Buildings and Public Parks of the National Capital) served as executive officer of the commission, in which capacity he oversaw its land acquisition program. Subsequent stream valley and other additions to Rock Creek Park in the District, including Melvin C. Hazen Park, Soapstone Valley Park, and Pinehurst Parkway, were purchased by the commission over the next three decades. A 1926 amendment to the act retitled the body the National Capital Park and Planning Commission and gave it the additional purpose of developing "a comprehensive, consistent, and coordinated plan for the National Capital and its environs in the States of Maryland and Virginia." 
This regional approach to parks and planning was motivated in part by awareness that Rock Creek and its parkland in the District would suffer increasing degredation if the creek and its watershed upstream in Maryland were not protected. In a 1913 paper on attractions around Washington, Lord Bryce, the British ambassador to the United States, had proposed park status for the upper valley on scenic and recreational grounds:
I should like to go even further [than the existing park]--although perhaps I am indulging in aspirations and not sufficiently thinking of appropriations--and consecrate the whole of Rock Creek valley for 10 or 12 miles above Washington to the public. It is a very beautiful valley Some day or other such a piece of scenery will be of infinite value to the people of Washington, who want to refresh their souls with the charms of Nature There are leafy glades where a man can go and lie down on abed of leaves and listen for hours to the birds singing and forget there is such a place as Washington and such a thing as politics within eight miles of him. 
In September 1925 Colonel Sherrill, as executive secretary of the National Capital Park Commission, wrote Gov. Albert C. Ritchie of Maryland to enlist his state's cooperation in park planning and acquisition for watershed protection as well as recreation. The governor agreed to appoint a committee to work with the commission. His appointees met the following May and named Maj. E. Brooke Lee, a prominent Montgomery County landowner and politician, as their chairman. In 1927 the Maryland legislature gave the committee legal status as the Maryland-National Capital Park and Planning Commission.40 Along with other land planning and development responsibilities, the Maryland commission would both acquire and administer most parkland in Montgomery and Prince Georges counties, the jurisdictions bordering the District of Columbia.
To inspire the District's neighbors to substantive action, the carrot of federal aid was deemed necessary. Rep. Louis C. Cramton of Michigan, chairman of the House subcommittee dealing with park appropriations, introduced legislation in 1929 that would have the United States grant one third and advance two thirds of the cost of extensions of Rock Creek and Anacostia River parkland into Maryland. On the House floor that December, Cramton announced that the Maryland-National Capital Park and Planning Commission had made detailed plans for the Rock Creek Park extension in consultation with the National Capital Park and Planning Commission and was prepared to repay the federal advance. The House passed the bill, and it was referred to the Senate's Committee on the District of Columbia chaired by Sen. Arthur Capper of Kansas. 
Colonel Grant, now executive officer of the National Capital Park and Planning Commission, worked closely with Cramton and Capper to resolve minor differences. As approved by the Senate and subsequently agreed to by the House, the bill specified a ceiling of $1,500,000 for the federal contribution and $3,000,000 more for the advance, to be repaid without interest in eight years. No appropriation would be made available until the two park and planning commissions had negotiated a satisfactory agreement on sewage disposal and storm water flow in the watersheds with the Washington Suburban Sanitary Commission, the bi-county sewer and water authority. The Capper-Cramton Act, as it was known, received President Herbert Hoover's signature on May 29, 1930. 
Conrad L. Wirth, a young landscape architect on the staff of the National Capital Park and Planning Commission, worked with the Maryland commission and completed a report on the extended Rock Creek Park boundaries that September. The following August the commissions entered into the required sewage and storm water agreement, and the Maryland commission accelerated its extensive land acquisition program. It was already at work continuing Beach Drive 1.2 miles north to East-West Highway. The Maryland portion of Rock Creek Park would ultimately reach upstream 22 miles from the District line and encompass 4,193 acres, as compared to 1,754 acres in the District portion.  Its separate administration would not be perceptible to most users entering and continuing along Beach Drive or a hiking or horse trail, and it would closely complement the federal park.
Two significant tracts in Georgetown might be considered additions to the Rock Creek and Potomac Parkway, except that one predates it as parkland. Montrose Park, occupying some 16 acres at 30th and R streets, owes its existence to a 1911 District appropriations act provision directing its purchase and inclusion in the District park system.  The force behind the legislation was Sarah Louise Rittenhouse, a Georgetown citizen determined to save the land from commercial development. The park became contiguous to the parkway when the land for the latter was acquired in the valley below. Adjoining Montrose Park on the northwest is Dumbarton Oaks Park, which Mr. and Mrs. Robert Woods Bliss deeded to the government in 1940 when they gave the main portion of their Georgetown estate to Harvard. It comprises 27 acres of wooded land with a stream valley descending to Rock Creek. Both parks afford convenient access to parkway trails from upper Georgetown.