The broad valley in the gap is considered outsized for the river it now carries, the Chehalis River, theorized to be due to its enlargement during the Pleistocene ice age when it carried meltwater from mile-high glaciers in what is now Puget Sound.

[9][10][11] The gap is responsible for climate and meteorological effects in Puget Sound region, including the Puget Sound Convergence Zone,[12][13] marine push,[4][14] diurnal wind circulation (sea breeze),[15][16] and the relatively high precipitation compared to areas in the Olympic Mountains' rain shadow.

[20] Municipal water supply in the Puget Sound metropolitan area is dependent on the rainfall which is in part attributed to the presence of the Chehalis Gap.

The unique climate and topography of the Puget Sound region mean that water from weather systems from the Pacific, particularly winter storms, falls preferentially in southwestern facing, elevated areas exposed to moist Pacific air, especially where the gap causes an "anti" rain shadow.

[21][22][5][23] Seattle, and Everett and Bremerton all have municipal reservoirs in the hills surrounding each city, sometimes tens of miles (kilometers) away.