When the Des Plaines River reached its crest of 9.12 feet — the sixth-highest reading ever recorded — in Riverside around 4 a.m., Feb. 21, you would expected there to be widespread flooding and street closures in the area.

Instead, the area’s relief valve was — as it was during a brief mild spell in mid-January — Swan Pond Park, where water backed up behind an ice jam in the bend of the Des Plaines near the Riverside Public Library, which surged with destructive power.

The high water came in the wake of temperatures last week in the 40s and 50s, plus a soaking rain on Feb. 20. As runoff from the rain and melting snow swelled the river, the frozen river broke apart and formed large jams in the river bend in Riverside.

Water that backed up behind the ice jam spilled over into Riverside Lawn, flooding streets and properties there. At the same time, it burst over the Swan Pond retaining wall.

Water and huge blocks of ice overtopped the retaining wall along the river near the former site of the Fairbank Dam with such force that the ice blasted through the top of the wall in at least two places, scattering about two dozen concrete blocks on the walking path and beyond. Ice blocks slammed into tree trunks, stripping the bank from them and, in at least one case, ripping down a large limb along with the bark.

“The larger trees with the bark ripped off will probably be OK,” said Public Works Director Edward Bailey, “but we have some concerns about the smaller trees that may not have fared so well.”

Ironically, said Bailey, a number of ash trees in Swan Pond that had been marked for removal survived unscathed.

Bailey said the damage to the infrastructure and landscape in Swan Pond Park last week was “twice as bad” as the damage from the January event.

“It was a classic physics lesson of volume and speed,” Bailey said of the damage wrought by the water and ice.

As the water receded from the park in the following days and temperatures again began to plunge, a portion of the park was turned into a sheet of ice while massive ice blocks littered the park near the sledding hill and areas all along the asphalt footpath.

Some areas, particularly in the vicinity of the former dam site, suffered obvious visible erosion and damage. The path, which had been repaired after being damaged by flooding in April 2013, was not visibly damaged this time. But areas underneath the asphalt had been washed away, putting the integrity of the asphalt in danger.

“We do know there has been significant embankment erosion,” said Village Manager Peter Scalera, who pointed out that officials had been working on a plan to address the damage sustained in January when last week’s flooding made it worse.

“Until [the ice] is gone we won’t be able to assess how bad it is,” said Scalera, adding that the village has been in contact with the U.S Army Corps of Engineers, which completed significant renovation of the retaining wall in 2012 as part of the project to remove the Fairbank and Hofmann dams.

“When we do restoration, we will need to pull permits with the Army Corps,” said Scalera.

While the huge ice jam on the river that prompted the flooding has washed away downstream, Swan Pond Park remains a prehistoric landscape of giant ice shards, debris from trees and even the odd, frozen fish caught up in the flooding and then trapped when the waters receded.

One giant carp seen Sunday had come to rest, frozen solid, on the asphalt plaza area near the interior WPA-era wall.