Just returned from an edifying visit to the Great Central Valley of California. As can be seen from the image above, the Central Valley is an enormous flat basin bordered by rounded hills on the west side and the rugged Sierras on the east (the photo above looks west along the flat plains towards some of the western hill country; the valley stretches for many miles to the east behind the photographer).

This huge flat tub contains some of the most fertile soil on earth. According to the hydroplate theory of Walt Brown, after the global flood event the continents slid and buckled and thickened, and as they rose in height floodwaters poured off of them. However, waters were trapped in many inland seas.

As described in the 7th edition of Walt Brown's book:
Drainage of the waters that covered the earth left every continental basin filled to the brim with water. Some of these postflood lakes lost more water by evaporation and seepage than they gained by rainfall and drainage from higher elevations. Consequently, they shrank over the centuries. A well-known example was former Lake Bonneville, part of which is now the Great Salt Lake.

Through rainfall and drainage from higher terrain, other lakes gained more water than they lost. Thus water overflowed the rim of each of these lakes at the lowest point on the rim. The resulting erosion at that point on the rim allowed more water to flow over it. This eroded the cut in the rim even deeper and caused even more water to cut it faster. Eventually, the entire lake dumped through a deep slit which we today call a canyon. [. . .]

With thousands of large, high lakes after the flood, and a lowered sea level, many other canyons were carved. Some are now covered by the raised ocean. It appears likely that (1) the Mediterranean "Lake" dumped into the lowered Atlantic Ocean and carved a canyon at the Strait of Gibralter, (2) "Lake California" filling the Great Central Valley of California carved a canyon (now largely filled with sediments) under what is now the Golden Gate Bridge in San Francisco, and (3) the Mediterranean Sea or the Black Sea carved out the Bosporus and Dardanelles. 107.
Examination of any terrain map of California confirms the likelihood of this theory. The richness of the soil there appears to support this theory.

The presence of a major submarine canyon under the ocean down the continental slope from Monterey Bay also appears to be supporting evidence for massive flows of water at a time when the ocean levels were lower (see image below from Google Maps -- this is a very distinctive piece of undersea terrain).

Submarine canyons such as this one (the Monterey Canyon) pose a problem for conventional (non-catastrophic) geological theories. As Dr. Brown explains elsewhere:
The ocean floor has several hundred canyons, some of which exceed the Grand Canyon in both length and depth. One submarine canyon is three times deeper than the Grand Canyon. Another is ten times longer (2,300 miles), so long it would stretch nearly across the United States. Most of these V-shaped canyons are extensions of major rivers. Examples include the Amazon Canyon, Hudson Canyon, Ganges Canyon, Congo Canyon, and Indus Canyon. What could gouge out canyons 15,000 feet below sea level? Perhaps ancient rivers cut these canyons when the ocean floor was higher or sea level was lower. If so, how did that happen? Swift rivers supposedly cut most continental [above ocean] canyons. However, currents measured in submarine canyons are much too slow, generally less than one mile per hour. Frequently, the flow is in the wrong direction. Submarine landslides or currents of dense, muddy water sometimes occur. however, they would not form long, branching patterns that characterize river systems and submarine canyons. Furthermore, experiments with thick, muddy water in submarine canyons have not demonstrated any canyon-cutting ability. 92.
The pattern of the Monterey Canyon certainly appears to bear the above analysis out.