The intersection of S. Walnut St. and W. Prospect Ave. forms a grassy triangle. In 1889 an open water reservoir was constructed on this site. The July, 1889 Appleton Post reported:
"The [independently owned] Water Works Company has commenced work on the excavation of its reservoir upon the old Webb place. The large trees which covered the lot have been cut down, and plows and big horse shovels are at work."
By 1912 the city owned the Water Works. According to the City of Appleton website: [In 1914] "The old reservoir above the plant (on Prospect Avenue) was refurbished and covered to provide 2.5 million gallon capacity for high pressure filter washing, fire protection, and added supply volume."
Water flowed from the reservoir to the water treatment plant directly below on Water St. "The Water Street Filtration Plant, constructed in 1914, remained in continuous operation with periodic upgrades until it was replaced with the 2001 treatment plant construction at a site located near the Lake Winnebago raw water intake pumping station." (City of Appleton website.) The construction of the new treatment plane meant the Walnut/Prospect reservoir was no longer in use.
In 2007 the late Walter Kalata, Alderman for the district including the Old Third Ward, proposed the following resolution:
“Resolved, the triangle shaped parcel of property located at Prospect Avenue and Walnut Street, site of the former water department water storage facility, be dedicated as a public park. In commemoration of the Appleton Sesquicentennial celebration and the historic fact that much of Appleton’s early settlement occurred in that Neighborhood, the new park be named, Pioneer Park.”
The resolution was later amended to read:
“Resolved, the triangle shaped parcel of property located at Prospect Avenue and Walnut Street, site of the former water department water storage facility, be dedicated as a public park, contingent upon approval by the Utilities Committee and Board of Public Works.”
While this official park was never dedicated, the neighborhood enjoys taking advantage of this little green space. At the tip of this triangle is a memorial to Frank Council, the first President of the Old Third Ward Neighborhood Association.
The Summer, 2003 issue of the Old Third Ward Neighborhood Association newsletter carried an article about the reservoir, including a reproduction of a photo of a boat being lowered into the water tank for inspection.
In 2011 the 
water treatment plant building on Water St. was demolished. At that time the pipes running from the reservoir down to Water St. were grouted solid.
Demolition of the reservoir was begun in 
April, 2013.
