Seawater is spilling onto driveways, flooding streets and creeping toward coastal homes as king tides push ocean water farther inland along parts of the U.S. coast this week.

In Southern California, flooding forced the closure of a stretch of Pacific Coast Highway near Huntington Beach, where people were seen wading through knee-deep water and navigating flooded areas on paddleboards.

In San Diego, a tide gauge topped 8 feet, nearly 1.5 feet above the predicted level.

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Outside of San Francisco, kayaks became the preferred mode of transportation as residents traversed inundated roadways.

Florida had its own high-water headaches.

Flooding covered roads near Fort Lauderdale and South Hutchinson Island, while Virginia Key recorded its highest king tide and third-highest water level in roughly 30 years of records.

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(MORE: South Florida swamped as Sunshine State sees "highest king tide ever”)

So, what's causing this?

King tides are the highest of the high tides, caused by a combination of the Sun's and Moon's gravitational pull. When the Sun, Earth and Moon line up during a full or new moon, their gravitational forces combine to create higher high tides. If the Moon is also near its closest point to Earth, those tides can climb even higher.

(MORE: How the moon impacts coastal flooding)

Weather can also give already-high tides an extra shove.

Onshore winds push seawater toward the coast, while powerful waves can carry water over beaches, seawalls and dunes.

Heavy rain can make matters worse by overwhelming drainage systems just as rising ocean water makes it harder for floodwater to escape.

In California, warmer ocean water associated with El Niño can also contribute to elevated sea levels, adding to the coastal flooding threat.

What to expect next

The water may begin to recede as the peak tides ease, but the coast isn't necessarily in the clear. Lingering onshore winds, rough surf and additional high tides could keep flooding and beach erosion concerns alive into early next week.

(MORE: El Niño brings floods to Peru, conditions expected to get worse)

And there's another round on the horizon.

California's next scheduled king tide window is during Thanksgiving week, followed by another round on Christmas, Dec. 23-25. 

For coastal residents, that means the water that reached the driveway this week could be back before long.