You should notice a pattern of rapidly varying water height in the Lake Mead chart above from 1935 until the mid-1960s, after which the water level became more consistent in the short term. My theory is this smoothing was caused by the fact that Lake Powell, upstream from Lake Mead, began to fill in 1966, taking 17 years to fill completely (that would take us to 1983). It is reasonable to assume the people overseeing this filling operation took more water for Lake Powell at times of rapid flow, thus smoothing out the flow peaks and troughs that were seen in Lake Mead beforehand. Since that time I would guess that Lake Powell now absorbs the annual peaks and troughs once seen in the Lake Mead data, and acts as a buffer for Lake Mead. I would love to confirm this theory, but there seems not to be a convenient monthly water level database for Lake Powell, as there is for Lake Mead.