Dam Diving at Lake Henshaw: Relocating a Discharge Tower in Black Water
A storm tore the marker buoy off the discharge tower at Henshaw Dam, and the mooring line went with it. What was left was a submerged concrete structure at the base of a working dam in San Diego County, with nothing on the surface pointing to it and no way to see it from below. Myers Marine Division was brought in to find the tower, rebuild the mooring, and put the marker back where it belonged.
The Structure Was There. Nobody Knew Where.
The discharge tower at Henshaw Dam had lost its surface marker in a storm, and with underwater visibility under six inches, no one on site could locate the attachment point again. Myers Marine Division was contracted to find the structure, fabricate a replacement mooring, and reattach it. The tower's position was known approximately. Its attachment hardware, 65 feet down in opaque water, was not.
Henshaw Dam is a 1923 earth embankment, 123 feet tall and 650 feet long, holding back the headwaters of the San Luis Rey River at the southeast base of Palomar Mountain. The reservoir behind it covers roughly 1,140 acres. A discharge tower in a reservoir that size is a small target, and the water at Lake Henshaw carries enough suspended sediment and algal load to shut down conventional commercial diving search methods entirely. Circle searches and jackstay patterns work when a diver can confirm a landmark by touch. Here the bottom was soft, featureless, and the structure could be missed by ten feet without anyone knowing.
That marker is not decorative. It is how the operating agency finds the discharge under service conditions. If the outlet becomes obstructed and crews cannot locate it, the stored water behind the dam stops being available to the population it serves and to the wildlife habitat that depends on the reservoir.
Locking Out a Dam Before Anyone Gets Wet
Before any diver enters the water at a dam, every source of suction and pressure differential has to be mechanically neutralized and locked. On this project the Myers Marine Division team entered the gear house first, closed the outlet gearing, applied locks and tags, and secured the gear house door itself so no one could walk in and open the dam with divers below. Nothing goes in the water until that is verified.
This is the hazard that kills divers at dams. An open or partially open outlet creates a differential pressure across the intake, and the force at a delta-P source scales with the area of the opening. A diver drawn against a discharge screen cannot pull free, and no amount of surface tending recovers them. The control is procedural, not physical: energy isolation under OSHA 29 CFR 1910.147, applied to every gate, valve, and gearbox that can move water, before the dive plan is signed off.
The dive itself ran under OSHA Subpart T and ADCI Consensus Standards using surface-supplied air, with a tended diver on umbilical, hardwire communications to topside, and a standby diver dressed and ready. Getting the crew to the water was its own problem. The dam face is steep enough that Myers Marine Division parked the mobile dive unit on the crest, geared divers up at the top, and rappelled them down the embankment on ropes to the waterline.
Seeing With Sound in Six Inches of Visibility
Myers Marine Division located the tower using a Blueprint Subsea Artemis diver-held multibeam imaging sonar, operated by the diver in the water rather than from the surface. In visibility under six inches, the diver held the unit's display directly against the faceplate of the dive helmet to read it. The sonar returned a full three-dimensional image of the structure ahead in water where a dive light showed nothing but backscatter.
Multibeam imaging sonar does not care about turbidity. It builds an image from acoustic returns, so sediment that stops light cold has almost no effect on the picture. What it produced here was not a blip on a bearing but a rendered form the diver could recognize as the tower, approach on a heading, and close on until it was within arm's reach. That is the difference between diver-held sonar imaging and a conventional search: the diver navigates to a structure he can see acoustically, instead of sweeping a pattern and hoping to bump into it.
Once the diver had hands on the tower at 65 feet, the recovery sequence was straightforward. He secured a small float to the structure with a messenger line and sent it to the surface. A small boat carried the assembled chain-and-buoy mooring out to the messenger, which was used to run the new mooring down the line to the waiting diver. He fastened it into the structure through a swivel, so the buoy can rotate on wind and current without putting torsional load into the connection or hockling the line.
Why the New Mooring Ends in Twenty Feet of Chain
The original mooring failed because it was polypropylene line for its entire length, top to bottom. Polypropylene is among the most ultraviolet-sensitive of the common rope materials, and a line left on a reservoir surface at 2,723 feet of elevation takes sustained UV exposure year-round. It does not stretch and part under load the way a worn line does. It goes brittle, then it snaps, and the buoy floats away. That is what a storm finished off here.
Myers Marine Division rebuilt the mooring with 20 feet of chain at the top end and poly line for the remainder of the run. The logic is simple: ultraviolet does not penetrate meaningfully past the first few feet of turbid water, so the only part of the assembly at real UV risk is the section near the surface. Twenty feet of chain is a conservative margin that puts the entire vulnerable zone in a material UV cannot degrade.
- Chain leader at the surface end, where UV exposure is total
- Poly line below the chain, in water that blocks UV
- Swivel at the structure connection, to keep rotation out of the line and the hardware
- Sized so the marker sits over the discharge under normal pool variation
The same reasoning applies to any marker or mooring on an open reservoir, and it is the kind of detail that separates a mooring that lasts from one that needs a dive team to replace it. Myers Marine Division handles this class of underwater construction and rigging alongside inspection and repair work at reservoirs across Southern California, including valve replacement at depth at Lake Jennings.
Frequently Asked Questions
Can divers work on a dam while it stays in service?
Yes, provided every energy source that moves water is isolated and locked out first. Myers Marine Division enters the gear house, closes and locks the outlet gearing, tags it, and secures the gear house door before anyone enters the water. The reservoir stays full and the dam stays in service. What gets shut down is the ability of anything to move water while divers are below.
How do commercial divers find a structure in zero-visibility water?
With sonar, not with lights. A diver-held multibeam imaging sonar builds a picture from acoustic returns instead of light, so suspended sediment that reduces visibility to inches has little effect on the image. The diver reads the display at the faceplate, identifies the structure, and swims a heading to it. Conventional circle and jackstay searches are far slower and can miss a small target entirely.
Why do marker buoys on reservoirs keep failing?
Usually because the mooring is built from polypropylene line for its full length. Poly is highly UV-sensitive and embrittles under sustained sun exposure at the surface, then parts under storm loading. A chain leader through the top 20 feet moves the UV-exposed section into a material that will not degrade, while poly below the waterline stays protected because UV does not penetrate turbid water.
Does Myers Marine Division take dam and reservoir work outside its core counties?
Yes. Myers Marine Division is based in Norco and works routinely across Los Angeles, Riverside, and Ventura counties, but the dive unit is fully mobile and inland reservoir and dam projects regularly take the crew further afield in Southern California. Equipment travels on a trailer-mounted mobile unit that can be set up on a dam crest without local dive infrastructure.
