The Santa Ana River Trail tunnel under the 405 will be closed from 7am Monday July 6 through 7am Monday July 13 due to construction for the 405 widening. OCTA reports that you'll be able to detour around the closures on the west side of the river, though note that portions of this detour have historically been unpaved (albeit hard-packed soil that our resident skinny-tire enthusiast reports is rideable with a bit of care).
Here's OCTA's map of the closure and suggested detours:
We note that OCTA didn't provide suggested detours for those using the river trail to bike between Fairview Park, Adams, Gisler, and Moon Park in Costa Mesa. So, in case OCTA does indeed shut down the entire length of SART as indicated (we wonder if they will just close the tunnel, and leave the rest of SART open south of Moon Park), here's a map to help navigate around the SART closure between points within Costa Mesa:
Briefly, from north to south:
California Street (Moon Park entrance of SART)
Minnesota (California to Gisler)
Gisler (Minnesota to Country Club; also connects to the Gisler SART entrance)
Country Club (Gisler to Mesa Verde West)
Mesa Verde West (Country Club to Placentia; if you want to reach the Adams entrance of SART from Mesa Verde West, we suggest Boa Vista to Balearic and then Shantar)
Placentia (Mesa Verde West to Fairview Park)
Fairview Park (Placentia to the Santa Ana River Trail)
We learned about this closure from an e-mail sent on July 2, 2020 by OCTA Bike which said that closure dates and times are subject to change, though they didn't link to a location where updates could be obtained. We'll update this page as we learn more.
Update July 8, 2020: OCTA confirms that the portion of SART from Adams Avenue to Moon Park in Costa Mesa is indeed open. Many thanks to Chris from OCTA's 405 program who sent us the following:
"The reason for closing the trail from Adams Avenue to MacArthur Boulevard is to ensure users do not reach the active construction area and have to turn around and retrace their path to reach the detour approved by OC Public Works. However, nothing prevents a user from accessing the trail at Moon Park or Gisler Avenue, for example, and traveling south. There is no access to the trail from the Sunflower/Cadillac path because of safety concerns."