That bucket brigade fascinated me also. Was there some of the backfill along the wall that was eroding? Did they fear a washout that could have taken out more of the spillway?
>>Bucket mystery discussion:
Given the volume of the buckets times the number of them in just these two photos, the ground would be bulging from the sum of the material. This indicates there is a “hole” that is being filled.
Now what type of “brown material” would you use to fill a “hole” - especially in a water sensitive situation? Is the hole a sinkhole? If so, this indicates something eroding or washing material away. I would/could believe that a flow could be going down towards the lower sidewall of the spillway. The hydraulic flow (or underflow) near/at the slab & sidewalls could be facilitating an evacuation of this “flow”.