By the late 1920s the capacity of the seven slow sand filters was found to be inadequate and so the output of the works was increased by the addition of three horizontal pressure filters, to which a fourth was added in 1938. This filtration capacity was to be reduced in 1960, when one of the slow sand filters was converted into a balancing tank for the supply of raw water to the Cumwhinton treatment works at times when these new works would be dealing with Geltsdale water. In 1938 an additional safeguard, namely chlorination, was included for the first time in the treatment following the Croydon Typhoid outbreak.

The treated water pipeline from Castle Carrock to Cumwhinton was increased in its carrying capacity at various stages to increase the volume of water it could carry so that, after World War II, there was duplication of the pipework over three quarters of its length.

In 1949 the drought made it difficult for Geltsdale to meet demand. This, coupled with the rising demand over the years, showed that the Geltsdale supply was by now inadequate. Three courses of action were considered, including the provision of additional storage either in Geltsdale or by raising the existing dam at Castle Carrock, the possibility of developing underground sources or the abstraction of water from the River Eden. It was this third one which was recommended. In 1953 the City Council applied for powers to abstract four million gallons per day from the River Eden at Wetheral. The Order was made on 30th June 1956 following a Public Inquiry.

Work commenced in 1958 on the construction of the River Eden Scheme as it was to be known. The works at the river comprised an intake constructed in the river bed with a pumping chamber in the river bank alongside. This chamber was connected to a further duplication of the trunk main from Castle Carrock to Cumwhinton over its remaining length, the now completed total length being converted into a raw water main. A treatment works was constructed at Cumwhinton and the trunk main distribution around the city was reinforced by a ring main around the south and west via a new pumping station at Harraby to deliver to a new five million gallon service reservoir at High Brow Nelson.

In 1960 the area covered for which the Carlisle authorities had responsibility was extended to include the water supply area of the Border Rural District. The inadequate sources and distribution systems inherited called for major capital works to be undertaken. It was recommended that only four sources of supply - namely Geltsdale, the Eden, Crew Fell and Roughton Gill - should remain and that all other small sources should be abandoned. The Crew Fell and Roughton Gill sources were stretched beyond their limits in both supply and distribution capacities so it was proposed and accepted that a major scheme, namely the North and Eastern Area Scheme should be connected to take treated Geltsdale water from Castle Carrock and supply a large part of the rural area in the north and east of the statutory area.

As the years went by, the extra demand for water in the Carlisle distribution area through the Cumwhinton works together with the allocation of water to the North and Eastern Area Scheme at Castle Carrock meant the increased use of the River Eden. Also the intake in the bottom of the river bed was proving unsatisfactory, with backwashing, a method of filter cleaning, becoming more frequent and less effective. It was decided therefore to seek additional water from the river, improve the intake facilities, and to increase the filter capacity and to improve the treatment generally. Approval was given to an additional two million gallons per day in any 100 days of the year and in 1972 a new intake was constructed in the river bank downstream of the first intake. Six additional pressure filters were added to the Cumwhinton works.