The Sioux River is a relatively large stream with a spring water source. It flows in a northeasterly direction and empties into Lake Superior. The main stream of the Sioux River is 13.3 miles in length. The Sioux River has two major tributaries. Four Mile Creek joins the Sioux 1.8 miles upstream from Big Rock, and the Little Sioux River joins the Sioux 1.8 miles below the Big Rock. There are also two small unnamed feeders that join the stream near its headwaters. All of these feeders are trout water and contribute greatly towards maintaining the high water quality.

BROOK, BROWN, and RAINBOW TROUT, and COHO SALMON are present, with BROWNS and RAINBOWS being more abundant. The Sioux River is considered to be Class IA, trout water from the County Highway "C" crossing downstream to Lake Superior, while upstream from the County Highway "C" crossing to its headwaters is Class IA. The Sioux River receives runs of COHO SALMON, RAINBOW and BROWN TROUT from Lake Superior. Both spring and fall RAINBOW runs occur, with the bigger run coming in the spring. The COHO SALMON and BROWN TROUT make only one spawning run each year around the latter part of August.

There exists a large deep hole below a wide flat layer of bedrock known as the Big Rock Hole. During periods of low water flow this rather steep layer of bedrock above the Big Rock Hole presents an obstacle to migratory trout, causing the fish to become concentrated in the pool below. In order to prevent snagging violations and mortalities in this pool, fishing in the Big Rock Hole is prohibited during the special spring and fall trout seasons.

The anadromous (lake run) RAINBOW TROUT enter the Sioux River in early April. They can be seen swimming up the Big Rock from the middle of April to the middle of May as they migrate upstream to their place of birth. The adult RAINBOWS average 21 inches in length, with males being slightly smaller than females. Males tend to be 3-4 years old at maturity and females are 4-5 years. RAINBOW TROUT are spectacular fighters and are highly prized by fisherman.

The anadromous BROWN TROUT enter the Sioux River in late August, with the peak migration being in October and early November. BROWN TROUT are similar to the RAINBOW TROUT in both age and average size. They tend to be more elusive and wary than the RAINBOW, and hence are harder to catch.

COHO SALMON enter the river at the same time as the BROWN TROUT, but the migration may last through December. They take on a very dark coloration (olive green to almost black) when spawning in the river. Adults are 3 years old at maturity, and die after spawning, unlike the RAINBOW and BROWN TROUT which are multiple spawners.

Big Rock Hole is a fish refuge (no fishing allowed) from September 1 through May 31 to help protect the anadromous run of BROWN and RAINBOW TROUT and insure that they will continue to migrate up "the rock" for future generations to enjoy.