We start with an unusually high snowpack for this time of the year (see graphic), with some regions at 200% of more of normal.
Snohomish - Skagit County line and includes Lake Cavanaugh. It drains 75 sq. mi. upstream of I-5, with the highest point in the basin just under 4000 feet. It is just under 20 miles from the remaining snowfields to I-5. The gauge data is from a Dept of Ecology site near I5 (see map)
The temperature of Pilchuck Creek also varied substantially (see below), with large swings of temperature during the warm days. The lowest temperatures matched those of the largest flows near midnight, with temperature peaking during the afternoon. On the the warmest day (May 28th), the stream's temperature varied from 14.2 to 18.2 C, a change of 4C (or roughly 7F)
What about the temperature of the water over a longer period? Here is the water year plot. The coldest temperatures were in early January (around 4C), with water temperatures zooming up to around 16C today. Keep in mind that bigger rivers can be much cooler.