The last time a storm washed them out was October 13, 2006. It was not storms plural but one huge storm. The people in the canyon who were camping out then said the water got 25 deep in the slot canyon known as Gold Strike. You can read their trip report at soak.net. Click on forum then click on Nevada. You will need to go back into the archives, however. It took my prodding and a couple of months before they posted.
A really unique story.