The native flora does little to help the soil retain water. During the intense rains of monsoon season or (relatively) wet months of January and February, a network of dry natural channels, called washes or arroyos, carved into the valley floor allows water to flow down from the mountains and converge in the Las Vegas Wash which runs through the Clark County Wetlands Park. The wash system used to form a large natural wetlands which then flowed into the Colorado River, until the construction of Hoover Dam on the Colorado River led to the creation of Lake Mead. Further development in the 1980s and 1990s made Lake Las Vegas, which required directing the Las Vegas Wash into tunnels which run under Lake Las Vegas and into Lake Mead.