What is the accessibility like? I have an elderly mother that uses a walker.
It depends on what you & your mother want to do. There are some great boardwalks and paved paths to observe the birds & alligators near the causeway. The castle within the property would also be accessible. Boardwalks lead to the beach which is gorgeous and quite firm near the water. My question would be what do you want to do at the beach? Having used a walker when I had a hip replaced I can tell you walking in sand is difficult & tiring...the type with wheels will have issues, but the basic, fold-up types don't have seats. You'd probably need to carry along a camp chair. There were benches on the trails and at the parking areas, but I don't remember seeing any once you got into the dunes & the beach beyond.
There are elevated walkways over the primary dunes, however, the beach is sandy and so your relative will have to deal with that. In general, I would characterize the accessability factor of Huntington Beach State Park as very good, perhaps not perfect, but again you are dealing with a natural beach.
There are a number of parking spots close to a number of nature trails, boardwalks, and vantage points for observing wildlife, including birds. There are ramps leading to some of the raised boardwalks. The Huntington Beach website is great so suggest you check it out. Unfortunately, I wasn't able to post the website for you. Just search Google as there is lots of great information.
The beach is huge but we didn't spend much time there so not sure how much the sand might make for difficulty for a walker.
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