The 1964 flood is the most studied hydrologic event in California history, and for good reasons: 1) it was the largest event that occurred during the era of quantitative hydrologic monitoring (~1900 – present), 2) it was the most costly in terms of property damage, and 3) it provided a valuable learning opportunity on how to build more flood resiliency into public and private infrastructure.

The Eel River experienced the most extreme flows among Humboldt County rivers in 1964. This graph shows peak flows exceeding 300,000 cubic feet per second (cfs) and corresponding peak river ‘stages’, or water surface elevations. Flood stage at Scotia is 51 feet, and most of these large events exceed that. Although the 1955 flood is often thought of as about equal in magnitude to 1964, the record shows the 1964 flood peak was nearly 40% larger than that of 1955. flood stage = 51 feet

Hydrologists have several ways in which to assess how large a past flood was, or future flood could be. These data show Eel River historical flood peaks and recurrence intervals, also called ‘return periods’. They represent the average number of years a flood of a certain magnitude will recur over the long term. While many characterized the 1964 flood as a ‘thousand-year flood’, it clearly was not, at least in hydrologic terms.

The volume of water delivered to our rivers and floodplains during the 1964 flood was staggering. All told, half a trillion cubic feet came downstream in a 10-day period. That’s enough water to inundate all floodplain areas within Humboldt County, including the Hoopa square, to a depth of 73 feet. Of course, not all that water came down at once. The Eel River stands out as the hardest-hit: although the Klamath watershed is four times larger than the Eel, it delivered less water.

This shows Eel River hydrographs and rainfall for the 1964 flood and the two largest storms since then, in 1974 and 1995, for comparison. 1964 was twice as large as the next in 1974, and six times larger than 1995. A second storm arriving on the heels of December, 1964, was not nearly as large, but came as another blow to an already devastated community.

Here we have the same hydrographs, but with an index of soil wetness called antecedent precipitation index, or ‘API’, which carries over and decays prior rainfall. The long duration of saturated soil conditions, along with denuded hillslopes from unregulated logging, contributed to the massive landsliding and resultant channel filling with sediment that marked the 1964 flood as among the worst in recorded history.

This view of Fernbridge, Ferndale, and the Humboldt Creamery in the aftermath of the 1964 flood conveys the feeling of helplessness those living through the storm must have felt. There is no doubt that floods of similar magnitude will happen again, especially in light of extreme weather events, both droughts and floods, predicted to be more frequent with the seemingly unstoppable march of climate change.