The Evergreen Aviation & Space Museum in McMinnville says the adjacent waterpark hasn't been keeping up with monthly payments it owes the museum, which has sued to collect and to compel the waterpark to make future payments.

It's the latest drama at the museum and waterpark, both started by Evergreen International Airlines' late founder Delford Smith.

They're now separate organizations but still attract families seeking to enjoy both the nonprofit aviation museum, whose collection includes the Spruce Goose, and the waterslides at the for-profit Wings & Waves Waterpark next door.

The two organizations have endured a succession of financial and legal setbacks over the past five years, beginning with Evergreen airlines' bankruptcy in 2013. The litigation suggests strains that could threaten the unusual, symbiotic relationship between the two organizations.

The museum and waterpark have a complicated but mutually dependent relationship. Companies controlled by businessman Steve Down own the waterpark and two museum buildings, but the museum does not pay rent.

In an especially unusual feature of the arrangement, the landlord (Down's companies, which include the waterpark) must pay the tenant (the museum) $70,000 per month. That provision recognizes the historical relationship between the two organizations, according to Robert Zeh, the museum's marketing manager.

In its lawsuit filed last week, the museum says the waterpark delayed payments in November and December and has paid only $11,000 of the $70,000 it owes for April. The lawsuit says the waterpark "unilaterally decided to pay other expenses instead of making payments to the Museum."

"It doesn't threaten our operations, but it is a large payment," Zeh said. "That's in the budget, and we're planning to receive that every month. It does create cash flow problems for sure."

John Neubauer, a manager with Down's companies, acknowledged the failure to make the April payment. He said the main reason is that it was undertaking deferred maintenance left undone by prior management that needed to be complete to keep the park open for spring break.

The original agreement to make those payments reflected an effort "to be a good corporate citizen," Neubauer wrote in an email.

Federal securities regulators investigated Down and his businesses in 2016 and 2017. It's unclear what the U.S. Securities and Exchange Commission was looking at, but Neubauer said word of the inquiry undermined Down's organization and was another factor its ability to make regular payments to the museum.

Evergreen timeline

December 2013

: Evergreen International Airlines files for Chapter 7 bankruptcy on New Year’s Eve. The airline had helped fund the Evergreen Aviation & Space Museum.

November 2014

: Delford Smith, founder of Evergreen and the associated museum and waterpark, dies at age 84.

2015

: Maine developer George Schott buys aviation museum property for $22 million with support from an aviation nonprofit called The Collings Foundation.

February 2016

: Michael King Smith Foundation, owner of Evergreen Aviation & Space Museum, files for bankruptcy. The filing averts a foreclosure auction.

August 2016

: Utah investment group The Falls Event Center buys the space museum, the Wings & Waves Waterpark and other nearby property. A businessman named Steve Down controls Falls Event Center and led the deal.

August 2017

: Aviation buffs learn The Falls Event Center quietly sold two of its vintage airplanes. The Oregonian/OregonLive reports federal securities regulators are investigating Steve Down and his companies, including The Falls Event Center.

September 2017

: Down files $25 million lawsuit against two Oregon men, accusing them of trying to sabotage his business.

April 2018

: Evergreen museum sues The Falls Event Center – which runs the waterpark – alleging it hasn’t made agreed-upon monthly payments.

Down bought the waterpark and nearby property in 2016. A relentless self-promoter, he bills himself as a "preeminent role model for 21st century business leaders" and casts himself as the museum's savior. But he's also come under fire because his company, The Falls Event Center, has sold some of the museum's airplanes.

Additionally, Yamhill County property records indicate the waterpark's owners owed nearly $280,000 in delinquent taxes last year. Neubauer said Down's organization is trying to resolve litigation over past county property reassessments.

"Upon that resolution, we intend to pay the balance of that still due," Neubauer said.

This article has been updated with comment from Steve Down's organization.

Oregonian/OregonLive reporter Jeff Manning contributed to this report.

-- Mike Rogoway; twitter: @rogoway; 503-294-7699