Base: Combine flour, baking powder, and salt. Cut in margarine. Mix in sour cream. Put into pan sprayed with non-stick cooking spray. Pat mixture into sprayed pan. Sprinkle with flour to spread.
Filling: Spread saskatoon berries over base. Top with sugar.
Topping: Combine sugar, flour, and margarine until crumbly. Sprinkle over fruit filling.
Bake at 350°F (180°C) for 30-40 minutes or until edges are nicely browned.