SB literally said in an earlier statement they didn't become concerned until 24-48 hours later, which is a direct contradiction to later statements. Honestly, I think the earlier statement was an attempt to shrug off the seriousness of the situation. If CL went to look on the 13th, which I don't believe was for 4 hours based on Fox 13's broadcasts that evening, and they went back the 14th and again on the 15th to get the car, that seems to imply some level of concern. However, he left his wallet and his phone and no longer had a vehicle to return home in and they didn't go back on the 16th, 17th, 18th, 19th, 20th, 21st or 22nd. That kind of implies a lesser level of concern when it should have been growing.
When SB was asked why CL hadnt participated in the search efforts earlier on, he commented something along the lines of the park had been closed to the public and he hadn't been able to search after that first time, but the park was open to the public until 9/23. There was time.
I don't know exactly when the flooding started, but it seems counterintuitive to go to the park 3 days in a row, remove his transportation, and then not return despite not hearing from him.