Potentilla anserina
This is sunny little flower almost always grows in wetlands or creeksides, though it can be found elsewhere. The photo shown here was taken on a seemingly-dry trailside, but because it was on a hill I wouldn’t be surprised if it is wetter than it appeared. Potentilla anserina is the most common cinquefoil in the area. In general, cinquefoils have a very distinctive “look,” with five spreading petals and surrounding a central mound of many stamens. The leaves are composed of crinkly, jagged-edged leaflets. Often the leaflets are arranged in many pairs along the stem (as with this silverweed here), but sometimes they are in threes or in a fan shape. Once you’ve seen a few different species the similarities start to jump out at you!
Flowers Of Marin 