CLEARLAKE, Calif. — Along cracked-mud shorelines and beneath tangles of weeds on parched riverbanks, artifacts and archaeological sites that have rested in peace beneath the water for hundreds and even thousands of years have become increasingly visible as the state’s drought has lingered, leaving them susceptible to plunder by opportunistic looters.

Though archaeological pillaging is age-old, the receding waters of the state’s lakes, rivers and reservoirs have lent special urgency to the issue here, exposing the remains of a lost California and alarming archaeologists, historians and especially Native Americans whose ancestral grounds are increasingly vulnerable to thieves bearing picks and shovels.

In recent months, a man was arrested twice for looting at Anderson Marsh State Historic Park here, where archaeological deposits date back 13,000 years or more. The initial arrest was made by Lake County detective, Richard Kreutzer, who had recently completed a class on identifying archaeological crimes, but even he was shocked by what was hidden in the suspect’s van: more than two dozen obsidian spear points, early stone hunting tools, a clay pottery bowl and other Native American artifacts, some with index cards specifying the depth of the soil in which they were found.