I don't recall seeing lots of old vehicles and docks at Decker Island, although we've anchored and fished in that area many times. (Maybe I wasn't paying attention). Just to be sure we're talking about the same place, leaving the launch ramp at Brannan Island state park, head along Three Mile Slough towards the Sacramento River. Once under the draw bridge, turn left into Horseshoe Bend. Decker Island will be on your right and the highway 160 levy road will be on your left. Here's the story:
The "island" was originally attached to the shore on the west side of the river and the river itself snaked around Horseshoe Bend, which was quite a bottleneck, both for boat traffic and for water flow. Around 1918 a cut was completed to bypass Horseshoe Bend, creating the Island. It's hard to believe when you see it today that the Island was originally part of the far (west) bank of the river.
For more information, refer to Hal Schell's Guide to Cruisng California's Delta, available from Hal's
web site. Rats, just read that it's out of print.
BTW the waters between the island and the west bank, on the main Sacramento channel, are prime striper fishing. Horseshoe Bend is good sturgeon fishing when they come back from the bay, but you'll also find them on the other side of the island.