Copper and Bacon creeks, both flowing from North Cascades National Park, merge into the Skagit from the right as it meanders slowly through an agricultural valley, past Marblemount, where the Cascade River joins from the left, and Rockport, where it receives its major tributary, the Sauk River, from the left.

It is crossed by Interstate 5, a major national highway, between Burlington and Mount Vernon; the four-lane bridge over the Skagit River collapsed in May 2013 and was reopened a month later.

The river supports one of the largest wintering bald eagle populations in the contiguous United States.

These geese feed on intertidal marsh plants such as bulrush and they are drawn to nearby farmlands where they find leftover potatoes in the fields.

The Skagit River was highly influenced by the repeated advance and retreat of the Puget Lobe of the Cordilleran Ice Sheet.

Ice and gravel moraines repeatedly blocked the Skagit, causing it to pool into lakes and forcing it to drain south into the future North Fork Stillaguamish River.

One of the several theories about this anomaly is that the upper Skagit once drained northward into Canada and the growth and retreat of successive Cordilleran ice flows brought about the reversal.

Each advance blocked the river, forcing it to find new routes to the south, in the process carving deep gorges.

Eventually, the Skagit gorge was so deep that even after the Cordilleran ice retreated for good, the river continued flowing south instead of north into Canada.

Archaeological evidence reveals that these peoples collected their food from the natural resources, through fishing, hunting, and gathering.

With two other American government men and ten locals from the Nooksack and Chilliwack bands, he canoed and portaged from the Canada–United States border down to Ruby Creek, a tributary of the upper Skagit River.

[7] Custer later talked about the area with an elder Samona chief named Chinsoloc who had lived there at one time; he drew a detailed map from memory, which the topographer found to be accurate.

The Skeetchestn Indian Band, of the Secwepemc (Shuswap) Nation, were located in the area of present-day Savona, British Columbia.

[7] Settlement along the river by European Americans in the late 1800s was inhibited by two ancient logjams that blocked navigation upriver.

[8] In November 1897, the Skagit River flooded severely; in the aftermath as the floodwaters receded, two new logjams formed and blocked navigation.

Using a recently built logjam removal boat named Skagit, teams finally cleared this jam in about a month.

[8] The years 1909, 1917, and 1921 are the other annual peak discharges of record for the gauging station at Concrete which is at the confluence of the Baker and Skagit Rivers.

The system includes 158.5 miles (255.1 km) of the Skagit and its tributaries — the Sauk, Suiattle, and Cascade rivers.