I just came across more information regarding this issue! Pretty interesting, and adds some details to the nature of this last clan battle. This is paraphrased from George Way of Plean & Romilly Squire's "Scottish Clan & Family Encyclopedia" (HarperCollins & Barnes & Noble, 1994, 1998)
George Sinclair, 6th Earl of Caithness, was forced to sell estates in 1672 to settle debts. He died without issue in 1676 and Sir John Campbell of Glenorchy claimed the title, being in possession of the mortgaged estates. He also married the widowed Countess! George of Keiss was a descendant of the younger son of the 5th Earl of Caithness and disputed Campbell's claim. Thus the battle ensued on the banks of the Altimarlach near Wick where, it has been said, "so many Sinclairs were killed that the Campbells were able to cross the water without getting their feet wet."
Just wanted to add to the story!