Feet of change.  This article says an expected rise of 36' to 877' above sea level.  The spillway of the dam is 884.  A dramatic change in height, but not anywhere close to a record.  

https://www.kcci.com/article/saylorville-lake-could-rise-more-than-30-feet-by-july-4-due-to-floodwaters-from-northwest-iowa/61244769