The lake, near Fairlie, was at 46% of its operating capacity at the end of last month.
Opuha Water has alerted the harbourmaster to the exposed tree and recommended recreational boaties keep their speed down in shallower parts.
The tree tops on inshore margins are at the south side of the lake.
Opuha Water said lake levels were being managed conservatively for farm irrigation, hydro-power generation, drinking water and recreational purposes.
The man-made 700ha lake has additional water beyond operating levels to allow minimum flows into the Opihi River.
Chief executive Bjorn Triplow said the lake was at the lower end of its normal operating conditions, but within set limits for the dam and lake to store water, irrigate farms and provide drinking water and power to the district.
More rain was needed to top up the lake.
Rainfalls last month in the upper catchment were useful for the lake and feeding rivers, but back on previous years, he said.
"Certainly we are in a different place than the same time last year when we had quite a few rain events over the December and January months and we were in a pretty good position at about 84%.
"We haven’t had the rainfalls of the last couple of years when we were blessed with significant amounts of rain to keep the lake levels a lot higher.
"I think 2014 was the only other year we’ve gone down below this mark, but 2014 was far, far worse than where we are at the moment."
Notices have gone up at boat ramps alerting boaties of the exposed trees.
Mr Triplow said submerged trees reappeared when the lake reached this level.
"If they are being too much of a hazard we will remove them if we have to, but only if they’re deemed to be dangerous."
He said an advisory group met often when lake levels reduced to monitor the lake’s environmental flows and available water for operational use to determine if restrictions were required.
Farmers avoided restrictions last month as demand for irrigation water was at 60% of the total take.
"If we see that demand change or increase, we may consider applying official restrictions to the farmers and that would likely be in the 25% range so we were well and truly under [last month’s] demand."
Irrigations restrictions were set last October at the start of the season as the water intake was below expected because the snow melt was down, and the prospect of an El Nino summer.
The dam generates electricity for the national grid for about 4000 households and supplies water to 16,000ha of farmland.
Power continued to be generated when water held in the lower dam was released during minimum flows.
In the past month, the lake level has fallen from 387 at a river stage height to about 382m compared with its full point of 391m.
