Put the clams in a bucket for 24 hours so they can purge (sand and muck). Then clean the clams. 95% of everything inside is good meat, especially the muscles that hold the shell shut. I cut away anything black. Then I pound the big pieces with a meat tenderizing hammer. 10 clams give about 5-6 cups of good minced meat.
Put them in food processor to chop them up fine.
In a skillet put garlic, onions, old bay, bread crumbs. Saute in olive oil, add parmesan, then spoon into the half shell.
Cook in the oven at 350 for 15 mins or so, until crispy on the edges.
They are good all year round in SD, since the water temps never get real warm like the east coast.
As far a runoff, they are bottom feeders and siphon off "cr#p" in the sand for a living, so I don't worry about how much it's rained.
Best way to get them is low tide, wade out to knee/waist deep water. Take a pitchfork, put it over your shoulder and walk backwards. when it feels like you hit a rock, push the fork in deeper, then lever out the clam. Took about 15 minutes to get 10 last Sunday.
Make sure you have a fishing liscense with a saltwater stamp, know the minimum size for keepers and don't take over 10!