What to know about California's failing Oroville Dam

Almost overnight, the Oroville Dam in California gained worldwide attention when authorities evacuated nearby towns, affecting almost 200,000 people, over fears that the emergency spillway was in danger of failing.

• Tens of thousands flee as California dam threatens to unleash floodwaters

The situation has since improved, but here are some things to know about what has happened and what could happen.

History

Lake Orovillelies east of the city of Oroville, 112 kilometres north of Sacramento. Construction of the dam began in 1962, and opened in 1967.

The lake is the second-largest reservoir in the state, at 210 metres deep andhas the capacity to store 4.3 billion cubic metres of water.

The dam itself is about230 metres high, about half the height of the CN Tower. Though many people think the Hoover Dam on the border of Nevada and Arizona is the tallest in the country, that honour belongs toOroville. Hoover falls just short, measuring 221 metres.

Canada's Mica Dam in British Columbia is even taller, at 240 metres.

How it got to this

It might be surprising to hear of flood advisories and evacuations in a state that has endured more than five years of drought conditions.

While the drought can't be declared officially over, conditions have certainly improved. California has seen above-average precipitation and the snowpack essential in providing groundwater has also vastly improved. As of Monday, the snowpack in the northern, central and southern Sierra was at a capacity of 179 per cent for this time of year.

As for Lake Oroville, which suffered severe drought conditions over the past few years, wet weather has brought it to 124 per cent of the historical average. More rain is expected over the next week, which could further complicate the situation.

The spillway

Initially, social media was on fire with reports that the dam was going to fail. But that is not the situation.

There are different types of dams: embankment dams, arch dams, gravity dams, buttress dams and versions that can incorporate qualities of both (the Hoover Dam, for example is considered an arch-gravity dam).In the case of Oroville, it is an earth-filled embankment dam.

Every dam has a spillway in case of higher than normal water in the lake. However, early last week officials discovered a huge hole in it about 76 metres long and 14 metres deep, according to the Los Angeles Times.

On Saturday, the emergency spillway was used for the first time in its history in an effort to lower water levels in the lake. But erosion around the spillway has authorities concerned that it could compromise concrete and cause an uncontrolled flow release of water.

If it were to fail, the spillway would run into Feather River below. Though the river itself is below flood conditions, a massive, uncontrolled flow of water into it could raise levels, endangering nearby towns.

As of Monday morning, water levels had dropped and officials were inspecting the emergency spillway. They hope to patch the hole before more rain falls in the region on Wednesday and Thursday.