In a heavy 2-quart saucepan over medium-heat, heat the sugar, corn syrup, salt and water to boiling, stirring constantly until the sugar completely dissolves. Stir in the almonds. Set a candy thermometer in place and continue cooking, stirring frequently, until the temperature reaches 300 degrees, or the hard-crack stage (about 15 to 20 minutes). As the mixture begins to darken, stir constantly.
Remove the saucepan from heat; stir in the butter and baking soda. Immediately pour onto the prepared cookie sheet. With 2 forks, quickly pull apart the almond mixture into a large rectangle.
Place the cookie sheet on a wire rack and cool completely. Use your hands to break the brittle apart into small pieces. Store in a tightly covered container.