This was the great tidal bore once more, which, at the occurrence of the spring tides, makes the entrance of the river extremely dangerous.  It is due to the narrowing of the Gulf of California forcing the tides into close quarters, and its violence is augmented by collision with the equally furious current of the Colorado.  The battle between this tidal wave and the Colorado continues for many miles, till at last the sea tide gradually loses its power and succumbs to the flood of the river.* The latter falls at the mouth, according to Ives, about thirty feet in a few hours after the ebb begins.  The shallower the water as the tide rushes in against the ebb, the angrier the wave becomes, sometimes reaching a height of ten or twelve feet.

* The tide ascends thirty-seven miles.  Lowest stage of water about three feet, average six feet, and highest about twenty feet.

At Robinson’s Landing, a mere mud flat, a camp was established and preparations made for the voyage to the extreme limit of navigation.  The parts of the steamer were put ashore and a suitable spot selected whereon to set her up.  The high tides were over for a month, and the mud began to dry, enabling the party to pitch their tents.  It was an uncomfortable spot for expedition headquarters, but the best that could be had, as the Monterey was not permitted by her owners to venture farther up the river.  But this delay, discomfort, and difficulty, to say nothing of expense, might have been avoided could a contract have been made with the existing steamboat company.  As the bank on which the boat was to be reconstructed was not likely to be overflowed more than a foot by the next high tide, a month later, an excavation was made wherein to build the steamer that she might certainly come afloat at the desired time.  Sixty holes had to be made in the iron plates so that the four stiffening timbers could be attached to the bottom to prevent the craft from breaking in two under the extra-heavy boiler.  Inside, cross timbers were also added to resist the strain.  On, December 17th, two steamers appeared from the fort, in command, respectively, of Johnson and Wilcox, to transport the army supplies to their destination.  Robinson, after whom the landing was called because he had a cabin there, was with the steamboats, and, as he knew the river, especially as far as Yuma, Ives engaged him for pilot.

By the end of the month, the Explorer, as the Ives boat was named, was ready for the expected high tide.  She was fifty-four feet long over all, not quite half the length of Johnson’s Colorado.  Amidships she was open, but the bow was decked, and at the stern was a cabin, seven by eight feet, the top of which formed an outlook.  For armament, she was supplied on the bow with a four-pound howitzer, though this weapon was not likely to be of much service.  When the anticipated flood arrived on the night of December 30th, steam was turned on at the critical moment, the engines worked the stern-wheel, and Lieutenant