Thunder Arm is the northwestern arm of Diablo Lake, a reservoir on the Skagit River within North Cascades National Park in Washington, USA. The primary and standard launch point for paddlers is at Colonial Creek, located off Washington State Route 20. This area provides a campground and a boat launch, though the ramp is noted to have silted in, making it suitable only for hand-launching kayaks, canoes, or paddleboards. Parking is available in a large lot above the ramp. The Thunder Arm Beach Trail, a very short walk, also leads to the water's edge. From the Colonial Creek launch, paddlers move down Thunder Arm to enter the main body of Diablo Lake. The water is famously a vibrant turquoise or milky aquamarine color year-round, a result of glacial flour suspended in the water. The scenery is alpine and grand, with steep, serrated mountains surrounding the lake. The paddling season is generally from late spring through early fall, as the highway closes in winter. Mornings are typically the calmest period on the water, with winds often picking up in the afternoon, sometimes exceeding 10 knots and creating waves, particularly when paddling the length of the arm. The water is glacier-fed and cold throughout the year. A key navigational feature is a boat passage barrier or boom that must be passed to transition from Thunder Arm into the main lake; paddlers should stay well clear of the dam structures at the far western end of Diablo Lake. Popular routes include paddling up Thunder Arm from Colonial Creek, exploring the inlets and shoreline, and continuing into the main lake. Boat-in campsites are available for multi-day trips, requiring a backcountry permit, and are on a first-come, first-served basis. The area sees other boat traffic, including a Seattle City Light ferry that shuttles to a dock on the main lake.