Along the shores of Lake Houston and Lake Conroe, many landlocked boat docks lead to nowhere while grassy islands and sandbars sprout where water once flowed.

Pieces of an old railroad, long hidden by the ample waters of Lake Houston, are suddenly visible.

Now both lakes may soon enter uncharted territory as the incessant drought and searing temperatures continue to deplete these two reservoirs' precious water supplies.

And things could grow drastically worse if no significant rain falls in the next week and a half, authorities warn. Each lake was built on the San Jacinto River as a source of drinking water for Houston.

The city of Houston this week has alerted the San Jacinto River Authority that it may have to take an emergency step that has not been done for two decades — order Lake Conroe to release up to 150 million gallons of water a day from its dam. The water would then flow downstream to Lake Houston, so that reservoir would remain deep enough to assure the city's water purification plant there can continue operating.

However, if Lake Conroe is drained of this amount for two months, the lake's water level will quickly plummet to a new all-time record low.

Conroe's current lake level is 197.3 feet above mean sea level, which is 3.7 feet below normal. The lowest the level has ever dropped is 5 feet below normal. That record was set in 1989, the only other time the city of Houston ordered water withdrawn for seven months, said San Jacinto River Authority's deputy general manager, Jace Houston.

With water levels nearly 7 feet below the normal 44-foot elevation on Lake Houston, boaters are more often hitting stumps or becoming stranded on sandbars that have surfaced.

Officer Gary Crawford with the Houston Police Department's Lake Patrol pointed to hundreds of old pilings now visible from a 2-mile railroad trestle crossing the lake from Huffman to Walden.

More for you News Falling lake levels, rising concerns

"We're working to cut them down below the waterline," he said. "But we could never tackle all the stumps. It would take an army to do that."

Different kind of summer

Duesen Park has also been forced because of insufficient water to close ramps that used to provide sailboats and kayaks access to Lake Houston. Residents around Lake Conroe are likewise unhappy that their lake level is hurting.

"The number of residents using the lake this summer seems to have dropped slightly because of the low level," said Texas Parks and Wildlife game warden Brannon Meinkowsky.

The shallower north end, he said, appears to be suffering the most.

"I don't have a lake now," complained Mike Bleier, president of the Lake Conroe Association. "I used to have lake-front property. But now I have a forest."

He cannot launch a boat or jet ski from his dock and gave up recreational use of the lake for the summer.

"It's dangerous to be out there if you're not familiar with it," he said. "Your boat prop can easily hit a stump or a sandbar now."

Nonetheless, Gary Lewis, TowboatsUS operator, said the lake continues to be populated by boaters.

"I don't see it as all detrimental, except for those who have boathouses that aren't usable," Lewis said. "I just see it as having more exposed sandy beaches to play on."

To secure the city's water supply, Houston taxpayers years ago paid for the construction of both lakes on the San Jacinto River. Lake Houston, covering 12,000 acres in northeast Harris County, began operations in 1953, followed by the 21,000-acre Lake Conroe in Montgomery County in 1973.

Lake Conroe is used to hold water in reserve until it's needed by Lake Houston's water plant.

"The water level on Lake Houston cannot fall below 37 feet or the water plant's intake pumps won't work," explained Alvin Wright, Houston's public works spokesman.

Down 3 ½ inches a week

The water level on Lake Houston currently stands at 37.6 feet, dangerously close to the mark that triggers withdrawals from Lake Conroe. The level is dropping by as much as 3½ inches a week.

"We're experiencing much higher water usage, while water pipes are also breaking from the extreme heat," Wright said. "Usually we have 250 repairs pending, but now we've got 600 repairs."

Plus, the municipal water supply is dwindling because of greater evaporation from triple-digit highs and no rain to replenish the losses.

"Houston's mayor and public works director will make the call to order any release from Lake Conroe. We're still hoping that won't happen," said Wright, who said the city wants to avoid dipping into its reserves until absolutely necessary.

However, he stressed that Lake Livingston, Houston's main drinking water supply, is nearly full, and the city has sufficient reserves to last two years.

cindy.horswell@chron.com