After copious amounts of rainfall in California in the last weeks, the artificial lake of the Oroville Dam is now filled far above its normal capacity. To hold the lake level at a maximum of 20 feet below the crest of the dam, the additional water can be discharged by a main spillway and an emergency spillway. Last week, cracks and a 300 feet wide hole suddentlyopened in the concrete of the main spillway. After a first inspection of the damage, a discharge test to lower the water level in the lake was undertaken, which caused more damage to the spillway. Reducing the discharge slowed down the damage done to the main spillway. However, Lake Oroville has been rising in response to the reduced outflow at the rate of about half a foot per hour in the last week.
Discharge test February 11, 2017 at the Oroville Dam. In the background the concrete weir and the emergency spillway are visible. The main spillway is already damaged and running water started to erode the left bank of the spillway. Image by William Croyle, California Department of Water Resource, work is in the public domain.
Discharge test February 11 at the Oroville Dam. In the background the concrete weir and the emergency spillway are visible. The main spillway is already damaged and running water started to erode the left bank of the spillway. Image by William Croyle, California Department of Water Resource, work is in the public domain.
As planned, as the lake level rises, the water pours over a concrete weir into the emergency spillway, which is parallel to the main spillway and has a capacity of 15,000 cubic feet per second. However, the actual inflow into the lake is still twice as much as the limited outflow. The unforeseen amount of water (the emergency spillway had never been used since the dam was completed in 1968) started to erode an area of earth downhill the weir.
Discharge test February 11, 2017 at the Oroville Dam. In the background the concrete weir and the emergency spillway are visible. The main spillway is already damaged and running water started to erode the left bank of the spillway. Image by William Croyle, California Department of Water Resource, work is in the public domain.
Discharge test February 11  at the Oroville Dam. In the background the damage done by erosion in the emergency spillway is visible. Image by William Croyle, California Department of Water Resource, work is in the public domain.
The actual danger here is that the headward erosion will erode the slope up to the base of the weir, causing the collapse of the concrete wall of the emergency spillway. (The Oroville Dam itself is at no risk of spillover or even collapse, however). This worst-case scenario would send a 30-foot-wall of water down the Feather River, a tributary to the Sacramento, flooding areas along the Feather and Sacramento River.
According to available geological maps, the Oroville Dam is build on the Smartville ophiolite, consisting of dark grey, steeply-dipping and strongly foliated metamorphic rocks. Once of volcanic origin, erupted in an ancient sea, the rocks were uplifted and metamorphosed during the formation of the Sierra Nevada.
Geological map of Lake Oroville. California Department of Water Resource, work is in the public domain.
Geological map of Lake Oroville with the Oroville Dam in the lower left corner and geologic legend. Rocks of the Smartville ophiolite in green, tectonic faults are depicted as black lines. California Department of Water Resource, work is in the public domain. Thanks to user @may_gun for the links.
This description seems to match the rocks visible in published photos of the main spillway damage. Some comments have also suggested that the first hole in the main spillway developed above an altered zone of rock, as its more brownish coloration would suggest. Altered rock is more permeable and prone to erosion than massive rock. It's possible that water infiltrated below the concrete of the spillway, eroding  the weakened rock until the concrete collapsed into the forming sinkhole. Which means that now running water can directly erode the bedrock, causing more damage (this possible explanation can however only be confirmed by a direct investigation of the exposed rocks).
Metamorphic rocks often cause engineering problems, as such rocks are often folded, resulting in tectonized/fractured areas in an otherwise stable rock mass. Also, the particular structure (the geological correct term is texture) of such rocks poses a problem. Metamorphic foliation forms when elongated or plate-shaped minerals (like micas) are forced by tectonic pressure into a same alignment. Between the minerals the parallel surfaces of the foliation form weak zones, prone to break. If such surfaces are tilted entire packets of rock can easily slip and be washed away by water.
With the emergency spillway the problem is similar, even if the erosion mechanism is not quite the same. The ground around the weir consists of rammed earth (soil consolidated with heavy construction machinery), but the ground of the emergency spillway consists of backfilled soil. This is a cheap and effective method to build dams, but it's also prone to erosion by fast flowing water, especially if the soil becomes soaked with it. Thus, the running water of the spillover is washing away soil, earth and rocks, forming deep gullies. The erosion front is creeping upslope as water flows down the slope.
The great problem now is that neither of the spillways can be used to control the water level in the lake without increasing erosion and damage in one of the spillways. During this storm, using the spillways as much as possible while trying to limit the discharge as much as necessary to slow down the ongoing erosion, the lake level was lowered by 8 feet a day and areas downhill the dam were evacuated as a precautionary measure.