Heron Lake Watershed District
Heron Lake State Dam Visit
September 22, 2016
Managers present: Harvey Kruger, Wayne Rasche, Bruce Leinen, and Jim Buschena
Managers absent: Gary Ewert
HLWD Staff: Jan Voit and Catherine Wegehaupt
Other: Randy Markl, Department of Natural Resources
The group met at the Heron Lake Watershed District (HLWD) office at 8:30 a.m. and traveled to the Heron Lake State Dam. Randy Markl explained how the hydrology of the Heron Lake system works, provided an historical overview of the dam, and described the management procedures for the structure.
The State Dam and a small parcel on the Heron Lake outlet is owned by the State of Minnesota, administered by the Department of Natural Resources (DNR)-Division of Waters, and operated by DNR-Section of Wildlife with cooperative agreements with the HLWD. Access to the dam for construction, operation, and maintenance is by easement over private property.
Heron Lake is approximately 8,200 acres in size and is separated into four sub-basins: South Heron, North Heron, North Marsh, and Duck Lake. All eventually drain through the State Dam. During flood conditions, the four sub-basins become one large contiguous basin. The lake receives its water from a relatively large watershed (over 300,000 acres), resulting in a watershed to lake ratio of over 36:1.
On August 12, 1932, the Jackson County Board of Commissioners established a Normal Water Elevation of 1399.6 ft. Mean Sea Level Datum (MSLD) for North Heron Lake and a Normal Water Elevation of 1400.8 ft. MSLD for South Heron Lake.
The Works Progress Administration constructed the original State Dam in 1937. Its main purpose was to conserve water after the drought years of the thirties. It was a poured concrete structure with nineteen, five-foot bays. The in-stream piers were set at an elevation of 1401.2. There were provisions for stoplogs in each bay. The stoplogs were installed at an elevation of 1399.6. Numerous times over the years stoplogs were installed above this elevation. The sill elevation of the structure was 1396.14.
In 1986 and 1987, the State Dam was rebuilt. The in-stream piers were cut down from 1401.2 to an elevation of 1400.2. Ten of the bays were filled with concrete to match the height of the cut down piers. Two of the bays were fitted with removable steel maintenance gates. The dam was equipped with a 40-foot wide hydraulic gate capable of being opened to any level from 1400.2 down to the sill elevation of 1396.2. When the gate is open, downstream channel restrictions control the lake’s outflow. If the gate is closed and the water is approximately one foot above the crest of the dam (approx. 1400.5 to 1401), the downstream channel again controls the outflow. Therefore, during high flow periods (floods) the downstream channel controls outflow, not the dam, regardless of the position of the gate. It has long been the opinion of some that the State Dam is responsible for periodic, extensive flooding. There is no scientific data to support this claim. DNR studies have shown that the dam has no effect during flood flows. The purpose of the State Dam modification in 1986 was to allow for the drawdown of Heron Lake and to hold water during drier periods, as needed.
The Dalziel Dam was located in the channel downstream of North Heron Lake in the North Marsh. It was given a permit by the DNR-Division of waters in 1940, though there may have been a structure of some sort at that site prior to this time. When surveyed in 1946, it was observed to be built out of wood planks and piling. Later surveys indicated it had a maximum crest of approximately 1400. The stoplogs and rocks in the bays were removed in December of 1989 to aid in the total drawdown of Heron Lake. This dam was completely removed in the spring of 1999.
In 2006, the concrete weir of the State Dam was cut down again from 1400.2 to 1399.5. This puts the dam at 3.3 feet high. It is only one foot higher than the runout elevation of South Heron and 1.5 feet higher than the runout elevation of North Heron.
Heron Lake was designated a Wildlife Lake by the DNR Commissioner in 1973. This designation allows water level drawdowns and water level management for the benefit of wildlife.
A channel system connects the sub-basins during low and average water levels. South Heron Lake’s deepest elevation is about 1395, with the deepest elevation of the north bay of South Lake is about 1397. South Heron Lake’s runout is approximately 1398.5.
North Heron Lake’s deepest elevation is about 1397. Its’ runout has lowered over the years and is presently about 1398.
As South Heron Lake dries out in a drawdown, water stops leaving the lake at 1398.5. The last 1.5 feet of water in the north bay of South Heron must evaporate if this portion is to dry out. Likewise, the last foot of water in North Heron must evaporate or be pushed out by winds. With the lakes’ various runouts, and the very large watershed, a total drawdown can only happen in drought years.
It has been noted at the water level gaging stations on the lakes, that water level bounces in the lakes, relative to rainfall amount in the watershed, are greater before and after the corn growing period. It might be assumed that if there was something growing in agricultural fields before and after corn, water level bounces would be lessened in that time period.
The site visit concluded at 10:00 a.m.