Record rainfall in the spring caused the spillway at the Murdo Dam to wash away and officials are still trying to decide what to do next to repair the dam.

The last 12 months have been the wettest in South Dakota in more than a century, and as a result dams across the state have suffered significant damage, creating the potential for flooding, loss of life, destruction of property and the need for expensive repairs.

Extensive snow melt and heavy rains have eroded spillways, plugged outflows, caused leaks and led a few dams to fail completely. The wet weather has exposed weaknesses in the state’s system of dams that in many cases are aged and worn.

A leak in the Quinn Dam near Philip in central South Dakota in March forced the closure of U.S. Highway 14, one of the state’s few east-west highways, while engineers determined the threat level. Murdo Dam, a 1930s-era Works Projects Administration dam just outside the town of Murdo in Jones County, saw its spillway severely erode this year.

State officials eventually determined that the Quinn Dam wasn’t in imminent danger of failure and re-opened the highway. Officials still are trying to determine the extent of damage to Murdo Dam and what to do about it.

“It seems like every year there’s one or two that go out,” said Ron Duvall, water rights permitting administrator for the Department of Environment and Natural Resources.

Estimates by the Association of State Dam Safety officials peg the cost of needed maintenance for the country’s more than 87,000 non-federal dams at roughly $66 billion. Around 15,500 of those dams, if they failed, could cause serious damage to property or could kill people. Just fixing those state-managed “high-hazard” dams would come at a cost of around $20 billion.

In South Dakota, there are more than 2,500 dams that qualify for listing on the National Inventory of Dams, meaning they hold more than 50 acre feet of water — about 16.25 million gallons — or have the potential to cause significant property damage if breached.

The maintenance problem has been slowly building over decades and it comes with another, potentially more expensive wrinkle. The lakes behind the dams are deteriorating too, placing the state’s multi-million dollar fishing and boating industries in peril.

South Dakota reservoirs support much of the state’s $270 million annual recreational fishing industry and its roughly $85 million annual boating industry.

Nearly all of South Dakota’s reservoirs were built during the first half of the 20th century. Many of them were built during the Great Depression when the Civilian Conservation Corps and the Works Progress Administration were created. Age and an increase in severe weather events have taken their toll on those depression-era dams.

The reservoir known as Lake Hiddenwood northeast of Selby was created by an earthen dam built in 1927. The dam washed out after heavy rains in the spring of 2018 and destroyed a bridge. The reservoir, owned by the Game, Fish & Parks Department, is home to a state recreation area; the park there remains closed.

Hidden Timber Dam, a WPA project east of Mission, was overwhelmed by snowmelt from back-to-back blizzards and heavy rains and breached this spring. What’s left of the dam still is owned by the South Dakota Office of School and Public Lands, which now is responsible for many dams built by public programs in the first half of the 20th century.

“It had a significant amount of silt in it,” School and Public Lands Commissioner Ryan Brunner said of Hidden Timber Dam.

GF&P now controls about half the CCC and WPA dams. Some of the remaining dams were built on federal land and are managed by federal agencies, but many more were given to the state for purposes other than recreation and now are managed by the office of School and Public Lands.

Dams that could kill people if they fail are designated as “high-hazard.” South Dakota has 91 high-hazard dams of which 44 are regulated by DENR. The remaining 47 high-hazard dams are owned, operated and maintained by various federal agencies.

Lakes all over the state, but particularly in the southeast, have seen their productivity and water quality slowly decline over the past few decades, said Jason Jungwirth, a GF&P fisheries biologist.

When built, the reservoirs provided places for fish to hide, breed and hunt. Reservoirs produced a lot of fish that people could catch. But as they’ve gotten older, many of the trees, bushes and rock piles have rotted away or been covered in silt.

As productivity and water quality decline, the lakes see less use. That can be a problem for nearby communities. Of the roughly $271 million anglers spend in South Dakota, only about $47 million was spent on the massive federally owned reservoirs along the Missouri River. Most of the spending is done around smaller reservoirs, many of which are owned by the state.

Jungwirth said GF&P has used fish stocking to help counteract the effects of age on state reservoirs. Stocking can be effective over the short term, but it masks the underlying larger problem and has kept the problem of aquatic habitat loss out of the public eye.

Jungwirth’s position was created in 2018 specifically to devise a long-term solution to the fish habitat problem. The process will be expensive and take decades to complete, but by the end of this summer, he said, the department should have a new aquatic habitat and access plan to guide future state efforts to improve fishing and water quality.

“Water is essential for all life,” Jungwirth said. “The better, cleaner the water is, the more life it’ll support.”

Lake Mitchell, on the northern edge of Mitchell, is a good example of what can happen when a reservoir gets old.

The 600-acre lake sits on the Firesteel Creek watershed, a roughly 360,000-acre area full of farms, ranches and feedlots. The Lake Mitchell dam was built in 1928, mostly to create a massive storage tank for Mitchell’s drinking water.

Like many of South Dakota’s reservoirs, Lake Mitchell has long collected the excess fertilizer, livestock wastes and loose topsoil that runs off of fields and pastures within its watershed. Since the 1970s, Lake Mitchell has sometimes closed to swimming and other forms of recreation that involve immersion, said Mitchell Public Works Director Kyle Croce. The closures have been due to toxic blue-green algae blooms and high concentrations of bacteria from runoff.

Runoff is a problem because once moving water hits the lake, all of the fertilizer, feces and dirt suspended in it settle to the bottom. The dirt builds up and makes the lake shallower, which means it warms up faster. The dirt also covers rock piles, flooded trees and other cover in which bass, bluegill, perch and walleye use to breed and feed.

The presence of fertilizer, and to a lesser extent animal feces, when combined with warmer, shallower water creates an ideal place for toxic blue-green algae to thrive. Blue-green algae isn’t actually algae; it is a cyanobacteria, one of the Earth’s oldest forms of life.

Cyanobacteria are one-celled organisms that use photosynthesis to make their food. They rely on many of the same sorts of nutrients as plants like corn or soybeans. Cyanobacteria also tend to grow in colonies. When the water is shallow, warm and chock full of phosphorus and nitrogen, cyanobacteria colonies become massive, floating, slimy, sometimes toxic blooms. The blooms can get so big that they block sunlight from reaching aquatic plants, which can lead to a reduced oxygen level in the water.

There are several types of cyanobacteria. When those bacteria die and their cell walls break down, those toxins — known as cyanotoxins — are released into the water. People and pets who come into contact with cyanotoxins can develop rashes, diarrhea, cramps and difficulty breathing.

The presence of cyanotoxins and fecal coliform bacteria, such as E. Coli, are two of the biggest causes for lake closures or listing by the state as unsafe for swimming, water skiing or any other type of recreation that involves water contact.

Lake Mitchell has been on the DENR list of impaired water bodies for several years, Croce said. It is just one of a growing list of South Dakota reservoirs that have problems with siltation and too many nutrients. Adding to the list of issues is an ever-shrinking amount of aquatic habitat.

Dredging the nutrient-laden silt out of Lake Mitchell would cost from $6 million to $10 million, Croce said. “For a small municipality in South Dakota, that is overwhelming."

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