The bank collapse had happened in the early morning hours, before first light, when the river was unwatched. A section of the clay bank on the western side of the channel, approximately twenty-eight meters upstream of the basin inlet, had failed along a horizontal shear plane about sixty centimeters below the bank surface.
The failure section was three meters wide and had deposited the entire volume of its clay and root-mass into the river channel in a single event, creating an irregular dam that extended across approximately two-thirds of the channel width.
The dam was not a complete blockage. Water was still flowing through the eastern third of the channel around the obstruction, but the reduced cross-sectional area had raised the upstream water level by an estimated four centimeters and significantly reduced the flow velocity into the basin.
More critically, the collapsed bank material was not stable. The clay mass was actively dissolving into the current, releasing fine silt particles at a rate that had already turned the water downstream of the obstruction from clear to a pale brown-grey color.
Zephyr reached the collapse site twelve minutes after he detected the current anomaly at the basin inlet. He hovered at head height above the obstruction, reading the air currents around the dam structure and observing the water flow patterns in the channel below.
The silt plume was visible as a gradual brown staining of the water surface, spreading downstream from the obstruction in a slowly expanding fan that would reach the basin within the next hour at the current rate.
The implications for the basin were severe and Zephyr understood them clearly. The diving beetles that maintained the basin's oxygen balance by circulating surface air to lower depths could not function effectively in heavy silt.
The caddisfly larvae constructed their protective cases from specific grain sizes of gravel and organic material that would be buried or displaced by a silt influx of this magnitude. The water lily root systems, already stressed by late-summer water temperature increases, were vulnerable to the reduction in dissolved oxygen that would follow a significant silt deposit smothering their root zone.
He observed the obstruction for seven minutes, flying a systematic survey pattern that took him from the upstream water face of the dam to the downstream scour zone where the reduced flow had already begun eroding the channel bed. The obstruction was not going to clear itself in any useful timeframe.
The clay mass was too cohesive to be broken up by the current alone, and the eastern flow path around it would simply maintain the silt plume until the entire dam dissolved — a process that could take three to four days during which the basin would receive continuous silt loading.
Zephyr turned and flew downstream at his maximum cruising speed, heading for Pebble Bend and the water strider colony. He needed help, and he needed it quickly.
The silt plume's color told Zephyr important information about the composition of the failed bank material. Pure clay silt produced a pale grey-brown suspension with a specific optical quality — translucent in thin layers, nearly opaque in thick ones, with a characteristic way of catching light that distinguished it from organic silt, mineral sand suspension, or algae bloom turbidity.
This suspension was pure clay, which meant the failed material was from the compacted clay subsoil layer — the kind of dense, cohesive material that would dissolve slowly but continuously, maintaining the silt load over days rather than clearing quickly the way a sand suspension would.
He assessed the downstream spread of the plume against the current speed and calculated a rough timeline. At the current dissolution rate and flow velocity, the silt concentration at the basin inlet would peak at approximately three times its current level in ninety minutes.
After that peak, the concentration would plateau until the dam material either cleared or stabilized. Without intervention, the plateau phase could persist for seventy-two hours — three full days of elevated silt loading into a system that was already under late-summer stress.
The water striders had detected the silt arrival at the basin surface before Zephyr returned from the dam site. They communicated this to each other through surface wave signals — rapid, short-wavelength vibrations transmitted through the water film at speeds that allowed information to cross the full width of the basin in under two seconds.
By the time Zephyr reached the basin inlet, the colony was already in its defensive consolidation formation, and the information about the upstream problem had propagated to every organism in the basin that was capable of receiving it.