I used these brackets for mine. http://www.canadiantire.ca/en/pdp/stationary-dock-side-leg-holder-0792631p.html#srp
Along with these for support. http://www.canadiantire.ca/en/pdp/stationary-dock-corner-leg-holder-1-8-in-0792621p.html#srp , http://www.canadiantire.ca/en/pdp/leg-brace-set-0792622p.html#srp
And they will sit in the middle of these that will then be filled with rocks.