Billion Gallon Border: Part II - The Old Mill Stream

In the last post, guest writer Robert Wilhelm began telling us the story of Hoopes Reservoir by first relating the tale of the Red Clay Creek reservoir that never was. In this post, he tells us of the background and construction of the reservoir that was created, and a bit of what once stood where the water now is.

-- Researched and written by Robert E. Wilhelm, Jr

Hoopes Reservoir, 1933

An alternate reservoir location involved Old Mill Stream, a tributary of the Red Clay Creek in Christiana Hundred and the location of the present day Hoopes Reservoir and Dam. A dam constructed across the Old Mill Stream valley could contain water at a higher level (~ 225’ water surface elevation above sea level) than a dam associated with the Red Clay Creek Valley (~180’ water surface elevation above sea level if a reservoir was to remain inside Delaware; 150-160 feet if not impacting Yorklyn). Holding 2-billion gallons of water, an Old Mill Stream reservoir offered greater water depth at the dam with equivalent storage capacity and less surface area as compared to a Red Clay water pool.

There was little infrastructure present in the Old Mill Stream valley which was not the case for the Red Clay Creek Valley. Within most of the 480-acre footprint of the proposed ‘Old Mill Stream Reservoir’ was the former mill property and farmland belonging to T. Coleman du Pont including Dupont’s summer home. Once an operating water-powered stone mill constructed in 1732, du Pont personally converted the idle mill into a mansion. A smaller stone mill constructed in the 1850s on the property was in ruins and would remain to succumb to the reservoir’s rising waters, while du Pont’s home would be demolished leaving only the stone walls behind.

Thomas Coleman du Pont and his wife Alice Elsie du Pont (his cousin) bought the property in 1906. By 1910 they had converted the original mill building to a rural weekend home to complement their home at 808 Broom Street in Wilmington. Called ‘Old Mill’ the original mill’s stone, quarried on the property, contained quartz, mica, and garnet speckles that the du Ponts left exposed on inside walls. The first floor, below grade and adjacent to the mill pond and millrace, contained the mill home’s water wheel powered electrical generating system. There was central heat with fireplaces in each of the major rooms. The second floor was assigned to the caretaker and family. The du Pont’s occupied the upper two floors with the third floor being open architecture for use as a living space or ballroom. Bedrooms and baths were on the fourth floor.

du Pont Old Mill residence

In keeping with the du Pont garden tradition, Mrs. du Pont planted both valley walls with daffodils, forsythia, magnolias, lilacs, roses, and iris. There were rustic bridges on pathways for viewing sculpture in the gardens. Mr. du Pont kept buffalo, elk, horses, and Holstein cows on the property as well. After the construction of Hoopes Dam and Reservoir, much of Mrs. du Pont’s Valley Garden which had become known as Old Mill Farm Garden or Old Mill Valley Garden, remained untouched high on the eastern side of the reservoir. Today, Valley Garden Park forms the core of a larger public park area accessible from Campbell Road (Route 82) which remains abundantly planted with daffodils, forsythia, and magnolias along the rolling pathways.

In the final analysis, Old Mill Stream was chosen as the preferred site due to lower completed project costs. Old Mill Stream offered fewer infrastructure conflicts, along with easier to obtain water rights given existing claims on the Red Clay by local landholders. Coleman du Pont, in poor health in his later years, was willing to donate his land to the City of Wilmington thus greatly reducing another project expense. The construction project, occurring at the time of the Great Depression, cost the City of Wilmington $3-million (nearly $52-million in today’s dollars).

Reservoir Construction

The topography of the Piedmont’s Old Mill Stream valley required the construction of a tall but narrow concrete masonry gravity arch dam. The dam’s integrity is assured by the Piedmont’s geology and geography. The location and design of Hoopes Dam insure the massively engineered structure remains sound. With construction beginning at the start of the Great Depression, the project became one of the largest and most significant public works projects in City of Wilmington and Delaware history.

The nearly 2-billion-gallon reservoir extends 1¾ miles (2.5 miles for a Red Clay reservoir) from the north end, where it is nominally 20-feet deep, to a dam near Barley Mill Road where the greatest reservoir depth of 105-feet is reached (Red Clay would have had a 50-foot deep pool at the dam). The reservoir stretches to about 900-feet at its widest point (a Red Clay reservoir would have stretched 2,500-feet wide in several locations).

The reservoir’s dam utilized 1930s era concrete dam construction techniques similar to the much larger Hoover Dam near Las Vegas, Nevada. Workers poured and formed large, interlocking concrete monolithic structures. Each monolithic block was cooled by a cold-water bath to prevent thermal cracking. Once completed, the concrete faces of the dam were shaped into a tall, smooth-flowing, boomerang-like arc. The structure rises 127 feet from the bedrock of the valley and spans 845 feet in length at the crest. Nineteen feet wide at the top and 94½ feet wide at the base (excluding the 30-foot spillway chute and apron). Anchored one-hundred feet into the bedrock walls and deep into the valley’s floor, the construction prevents leakage around or under the dam. A Red Clay dam would have been about 550’ wide and 70’ high with correspondingly smaller thickness dimensions.

ColouriseSG version of Black & White Hagley Museum & Library Image
of the Hoopes Dam & Reservoir Under Construction

On the reservoir side of the dam east of the spillway, is a square gate chamber connected to the pump house by a three-foot diameter pipe at the foundation. The gate chamber, rising the height of the dam, contains wire screen openings allowing for the filling or drawing of water from the reservoir. A pumping station at the base of the dam initially included three gasoline powered centrifugal pumps providing four million gallons of daily pumping capacity. Depending on the City’s need and the volume of water contained in the reservoir, water may be transferred to the reservoir from the Brandywine Creek or drawn from the reservoir to the City of Wilmington for municipal drinking water when the Brandywine Creek supply is supply limited.

The City of Wilmington monitors the level of the reservoir constantly and adjusts it as needed to limit overflow down the spillway and subsequently reducing Brandywine Creek demand. Only severe storm overflow pours over the spillway weir where the discharge cascades down the 0.6:1-foot sloped face of the dam collecting and losing its kinetic energy within a concrete apron. The flow is directed to a concrete box culvert following part of the Old Mill Stream’s original routing. Leaving the culvert, but still traveling the Old Mill Stream tributary, the discharge travels an additional 1,300 feet, passes under Barley Mill Road, and joins the Red Clay Creek.

Campbell Road (Route 82) crosses the northern end of the reservoir and valley. A causeway, constructed to raise the road surface above a full reservoir water level, includes a pair of five-foot by ten-foot double-box concrete culverts permitting water and underwater wildlife to pass between the smaller northern reservoir pool and to the much larger southern reservoir water pool.

ColouriseSG version of Black & White Hagley Museum & Library Image
of the Completed Hoopes Dam & Reservoir with Valley Garden Park

The dam’s design includes a walkway crossing the 25-foot wide spillway. The original plans included the Valley Garden Park south of the dam in an open area across from the pump house. After the project’s completion, visitors frequently enjoyed the public park and could hike a foot trail and steps leading to a war memorial and monument located on a viewing platform at the crest of the dam. Because of safety concerns, the park and access to the walkway have since been closed.

In 2008 the spillway was raised an additional two feet adding another 150 million gallons of capacity (the weir may be raised an additional 3-feet in the future if necessary). Hoopes Reservoir is Delaware’s largest northern reservoir and serves as an emergency water reserve for 60% of Delaware’s population. Today, visible from the International Space Station, Hoopes Reservoir is no longer open to the public and is managed solely as a reserve water storage reservoir. Private tours of the dam and access to a recreational trail leading to the top of the dam and the viewing platform may be arranged by contacting the City of Wilmington Commissioner of Public Works.