We took a road trip down the Great River Road last week, following the Mississippi down the Wisconsin side.
At a roadside stop we stumbled upon a grove of Black Walnut (Juglans nigra) trees, with oodles of windfall walnuts. Lucky for me! This tree is native to eastern North America, including SE Minnesota/western Wisconsin.
I filled my bag, which yielded approx. 6 lbs. of hulls, 5 lbs of which I froze (recommended by J. M. Niles). Some of them were almost as big as apples!
I soaked the hulls in rainwater along with a splash of vinegar overnight, simmered them and then dyed some wool and silk samples. Results later this week.
silver minnow 
those look wonderful and what a beautiful place to collect!