The Balancing Reservoir at Loch Raven

The Loch Raven Reservoir watershed property is a fascinating place full of relics of another time. An especially interesting place is the Balancing Reservoir, a decommissioned reservoir located near Cromwell Valley Park.

Take a look at the history and function of the remnants found at the Balancing Reservoir, and look at how I surveyed and added information about the reservoir to the map on

balancing shaft
The old shaft that conveyed water from a 12′ wide conduit built in 1881 to the reservoir above


Loch Raven Reservoir is a 23-billion gallon reservoir located near Towson, Maryland. The original reservoir was smaller and utilized a low dam which is still standing, downstream of the upper dam. The original lower dam connected with the Lake Montebello reservoir via a 12-foot diameter conduit. The conduit was built by tunneling underground and follows a nearly straight line to the Montebello gatehouse.

The construction of the upper dam at Loch Raven raised the water level of the reservoir from 170 feet above sea level (at the lower dam) to 188 feet and eventually to 240 feet. Raising the dam increased the water storage capacity significantly but also increased the water pressure in the 1881 conduit. The pressure presented a problem for city engineers. To reduce the water pressure in the conduit, city engineers elected to build a balancing reservoir over the conduit.

The Balancing Reservoir

How the balancing reservoir looked while in use. Via Ronald Parks.

The balancing reservoir allowed the excess water pressure in the conduit to be relieved. The facility consisted of a smaller reservoir (nine million gallons), which stored the over-pressure water in an abandoned quarry behind an earthen dam. Water would flow back into the conduit when the upstream supply decreased.

The balancing reservoir has been dry since 1946, but the earthworks remain. With a little imagination, a visitor can visualize how the reservoir worked. Thre are four main components that remain to help.

  1. The standpipe (or shaft), which conveyed water between the conduit and the reservoir.
  2. The dam, which impounded up to nine million gallons in the reservoir.
  3. The spillway, which conveyed excess water out of the reservoir, down an outfall and into Minebank Run
  4. The quarry, which provided the basis for the reservoir.

Finding the Remnants Today

The balancing reservoir became obsolete when a new tunnel opened between Loch Raven and Montebello in 1946. Though it fell out of use, all four components remain on the watershed property.

The Shaft

The shaft is technically known as a standpipe. It forms a connection between the tunnel below and the balancing reservoir above. The shaft extends from the top of the cap to the bottom of the tunnel, 55 feet in total. The exposed portion of the shaft extends 22 feet from the ground to the cap. While in operation, water from the conduit would rise through the standpipe and out one of the six 36-inch portals at the base.

A close-up look at the 36″ portals that cover the shaft.

The earthwork around the shaft is finished in concrete and sloped so that water can drain from the balancing reservoir back into the conduit. The outflow down the shaft would occur whenever the sluice gates at the Upper Loch Raven Dam were closed or were not feeding enough water to fill the conduit.

Today, the 22-foot tall shaft and surrounding concrete earthworks protrude prominently out of the earth in a wooded area just east of Cromwell Valley Park.

The Dam

A view looking along the footpath that crosses the earthen dam.

The earthen dam, clad in concrete on the inside face, is intact. The dam is 8 feet wide along the top and has a crest elevation of approximately 205 feet. The dirt used to construct the dam was excavated from the nearby hillsides.

Today, there is a dirt path that runs along the top of the dam, with a side path going down the face into the reservoir which leads to the shaft and continues to an unnamed dirt trail that connects the two parts of the Cromwell Valley Park.

The Spillway

The spillway is a twenty-foot wide concrete channel that drained excess water from the balancing reservoir, which had a low elevation of 190 feet. The spillway elevation is 200 feet at its peak, so water more than ten feet from the bottom would flow out of the spillway and into a channel that led to Minebank Run.

Today, the concrete paved channel is still intact and is a strange sight to see out in the woods. The channel is dry now, with a few trees growing through cracks. The outfall has a few danger signs to warn the public of the steep drop off beyond.

The Quarry

Panoramic photo over 180 degrees inside of the quarry

The old quarry was chosen as the site for the reservoir due to its proximity to the conduit. The abandoned quarry was also a desirable place to build a reservoir as the earth had already been removed. The quarry dates back to the 1860s.

Today, the quarry is plainly visible a few steps from the balancing shaft. There is a short path the goes into the horseshoe-shaped quarry where the hiker can see the chiseled rock all around. There are also remains of a fire pit in the quarry.

Mapping the Balancing Reservoir

The balancing reservoir area as seen on the Strava basemap.

OpenStreetMap is a map that anyone can edit. The data it houses can be downloaded by anyone as well. I enjoy adding data about parks and trails, mainly because others can use that data to plan hikes or trail runs via sites like Strava and GaiaGPS. These sites use OSM data to power their routing tools and maps.

The Balancing Reservoir area is covered by trees and does not look like anything special on aerial photos. Trails in the woods are challenging to map by tracing aerial photography. There are two methods that I use to add trails and other features to the map that are typically obscured by trees: LiDAR and GPS tracking.


Lidar is a technology that uses laser light to measure the elevation of a surface, typically ground elevation. LiDAR datasets are enormous in raw form, so they’re often rendered into map form for viewing. The State of Maryland publishes shaded relief maps for each county in Maryland on the state’s IMAP open data portal. The data is free from license and accessible to all at no charge.

The balancing reservoir stands out as a chiseled area in the center of the image

Based on the lidar, leaf-off aerial photography, and an old plan sheet for the balancing reservoir, I was able to identify the location of the shaft, spillway, dam, and quarry quite quickly. The surface of the earth does not tend to change drastically over the timespan since the reservoir was in operation. Additionally, moving dirt around is an expensive endeavor, so we can expect that the balancing reservoir will remain intact unless it becomes a grave concern to the public.

1:2000 scale shaded relief map showing the components of the balancing reservoir and the approximate location of the 1881 conduit


Some paths and trails are visible on the shaded relief map and aerial imagery. I added the paths that I could see on these maps, but ultimately a ground-truthing expedition was required add all of the balancing reservoir features.

I set out on a short hike to verify and photograph the balancing reservoir site facilities. While walking, I recorded my position with GaiaGPS on iPhone, and with a Garmin VivoActive HR watch. After the hike, I saved the GPS data from Gaia and Strava and loaded it into the JOSM OpenStreetMap editor for input into OSM.

The paths and features added to OSM via JOSM

Back on the computer, I brought in the GPS data (in the form of GPX files) and photos to JOSM. Switching between the lidar and regular imagery layers, and using the photos and GPS data for reference, I added the paths, cliffs, and spillway to map. The Lidar-based shaded relief map is especially helpful in this endeavor, as the spillway, dam and cut lands are plainly visible.

A static map showing the balancing reservoir on the default OpenStreetMap layer

Exploring the Balancing Reservoir

Static preview of an interactive map I made to show the locations of the trailheads and paths leading to the balancing reservoir

There are three points of entry to the balancing reservoir site, marked as trailheads on the map. The Lime Kiln trailhead is temporarily closed for renovations (as of December 2016). The Greenhouse Path trailhead is better for accessing the area from the Willow Grove Nature Center area, while the Sycamore Path trailhead is better for accessing it from the Loch Raven Drive area.


Special thanks to Ronald Parks for his insight into the history of this area. Ronald has published two books that document the history of the Loch Raven Reservoir and maintains a blog about the Baltimore City water system as well.

Thanks to the Baltimore County Department of Recreation and Parks and the Cromwell Park Council for publishing inspirational maps and other information on the Cromwell Valley Park website.