THIS CAN BE A VERY HAZARDOUS CACHE. DO NOT DO THIS ALONE. THERE IS A REASON ITS CALLED THE TUNNEL OF DOOM.
To access the Tidal Lake off of I-495 from the Christina River, you will have to go through a 250 foot long tunnel under I-495 that has the tidal flows to the large lake.
You should bring extra food, water, flashlights, and warm clothes. YOU COULD BE TRAPPED IN THE LAKE AREA FOR UP TO 6-8 HOURS IF YOU DO THIS WRONG. The tunnel will look plenty big and high, but at high tide there could not be enough headroom to make it through, With the tide comming in, it is very easy to go through the tunnel to the lake--no paddeling needed, the current will take you through very quickly, just be ready for the 20 feet of whitewater as the tunnel empties into the lake.
THE PROBLEM IS THAT YOU WILL NOT BE ABLE TO PADDLE BACK AGAINST THIS CURRENT until it greatly slows down at approaching high tide (when there is little head room), or you can wait even longer until the tide starts to go out, the headroom increases, and the current will take you out.
If you enter the tunnel at close to high tide, the good news is that the tightest headroom is at the river side so you will not get stuck towards the end of the tunnel.
If you enter the tunnel at a hour past low tide and have a high head clearance waterctaft, you could be stuck until it is an hour or two past high tide--thats an 8 hour wait.
It might be possible to enter the tunnel about 1-2 hour before low tide and make it up the tunnel against the flow that will be slow at that point, find the caches and get out before the tides turn. When I did it with a caching friend we entered during mid tide and had to wait awhile--past when it got dark and as we paddled out we had to get down very low in the kayak to not scrape your head on the top of the tunnel at almost high tide..