Fish Kill Event - Des Moines River

Background

The United States Army Corps of Engineers (USACE) owns and operates two flood control reservoirs on the Des Moines River. Saylorville Lake is located in Polk County and its primary purpose is to minimize flood impacts to downtown Des Moines. Red Rock Lake is located 45 miles downstream of Saylorville Lake, in Marion County, and it provides flood mitigation in Southcentral Iowa as well.

Gas bubble trauma (GBT), occurs when fish are exposed to water that is supersaturated with gases. GBT has been linked to fish kills in the tailwaters of both USACE flood control reservoirs on the Des Moines River, but primarily below Red Rock Lake. Lutz (1995) monitored gas supersaturation-induced fish kills in the Red Rock Lake tailwaters for over a decade (1983-1994), eventually linking GBT to periods of continued high dissolved gas pressures when reservoir discharge was substantially decreased. The USACE has utilized the results and altered operations to improve conditions and fish health in the Red Rock tailwaters and further downstream.

Environmental Conditions

The Des Moines River watershed received heavy precipitation in the spring/early summer of 2024. However, drought conditions returned to the Midwest late in the summer of 2024. A down ramping event in mid-September was particularly intense (Figure 1). In this particular case, flow was switched from the hydroelectric facility to the USACE side of the dam. This procedure actually requires a brief increase in flow to avoid dropping below minimum flows (300 cfs), followed by a rapid down ramping, which is a common practice when the outflow reaches approximately 1,300 cfs. (personal communication w/ USACE staff)

Total dissolved gas pressures in the Red Rock tailwater were also elevated in mid – late September (119.4 – 127.1%) following this operational procedure (Figure 2). These pressures surpassed the Environmental Protection Agency’s national criterion of 110 %, increasing the likelihood of a gas supersaturation-induced fish kill. Unfortunately, the period of elevated dissolved gas pressures coincided with intense down ramping, creating similar conditions to those historically linked with fish kills below Red Rock Lake (Lutz 1995). These conditions were suspected to cause the fish kill downstream of the Red Rock Lake Dam from September 11th - 14th, 2024.

Fish Kill

Carl Schoenfield (USACE) observed dead fish in the Des Moines River, at the Howell Station pedestrian bridge on September 11th, 2024. Additional reports were fielded by Iowa DNR staff from September 12th – 14th, 2024. Species included Common carp, Channel catfish, Freshwater drum, Hybrid striped bass, Gizzard shad, and Bluegill. At a minimum, there were hundreds, if not thousands of fish that perished over the 3-day period. Dead fish were reported from the dam, downstream to the Horn’s Ferry Bridge. Iowa DNR Fisheries staff were on the site on September 16th, 2024 and observed one dead Gizzard shad. Staff collected water chemistry data, including TDG, which indicated that total gas pressure was elevated at 123.6%.

Staff suspected GBT due to elevated total gas pressure. Staff returned with boat electrofishing gear on September 18th, 2024 to sample the tailwaters and examine captured fish for signs of GBT. Obvious signs of exophthalmia (a.k.a pop-eye) were present in multiple species, indicating that elevated total gas pressure and subsequent gas bubble trauma was likely the cause of this fish kill (Figures 3). Total gas pressure remained elevated during Iowa DNR and USACE sampling events on September 18th and 23rd.

Action Items

• • Continue monitoring both Saylorville and Red Rock tailwaters for additional fish mortality.
• • Identify total gas pressure levels (EPA recommends <110%) that trigger communication between USACE & Iowa DNR Fisheries staff.
• • Discuss modification of down ramping procedures to avoid gas bubble trauma below Saylorville & Red Rock.
• • Utilize continuous monitoring of total gas pressure to improve modeling accuracy of the relationship between environmental variables, operational procedures, and gas bubble trauma.
• • Incorporateoperational changes into the USACE and MRES operational protocols.