DNR to Lower Ventura Marsh Water Level This Fall to Kill Invasive Carp

VENTURA, Iowa — The Iowa Department of Natural Resources will begin lowering the water level in Ventura Marsh this fall to induce a winter kill of a common carp population that has reentered the marsh, the agency announced.

Common carp are an invasive species that harm water quality by uprooting aquatic plants and stirring up sediment from the marsh bottom, according to the DNR. As the carp population grew, water quality in Ventura Marsh declined and habitat conditions became severely degraded, affecting water quality in Clear Lake.

The DNR previously installed fish barriers to keep carp from entering Ventura Marsh to spawn as part of the Clear Lake water quality improvement project. Water levels were last lowered for management purposes in 2017, but higher rain totals in the years that followed caused water levels to rise and overwhelm the fish barriers, allowing carp to return to the marsh.

“When healthy, vegetation in Ventura Marsh acts as a filter, removing excess nutrients and sediment from the water before it enters Clear Lake,” said Scott Grummer, fisheries management biologist with the Iowa DNR’s Clear Lake Station. “A healthy Ventura Marsh helps maintain better water quality in Clear Lake. Nearly half of Clear Lake’s watershed passes through Ventura Marsh.”

Wildlife habitat conditions within the marsh have also become severely degraded because of the carp presence, and local fish and wildlife staff said they expect major improvements to habitat and water quality once the project is complete.

“These improvements should set us up for some really quality hunting and bird watching opportunities in the near future,” said Rob Patterson, an Iowa DNR wildlife biologist with the Clear Lake Wildlife Unit.

Grummer said weather will play a key role in the project’s success.

“One of the main challenges with any water level management project is too much rain,” he said. “Excessive rain during the project could impact our ability to get water levels as low as we’d like, so we’ll monitor the weather and be adaptive in our management. If the weather cooperates, we shouldn’t have many issues achieving the goals of the project.”

The lower water level will be maintained through next year’s growing season, according to the DNR. Refilling the marsh will begin as early as fall 2027.