This seems to be common milkweed (Asclepias syriaca). The so called butterfly weed (Asclepias tuberosa) has orange flowers and is a smaller plant, see

but A. syriaca doesn't occur in CA. I was too lazy to think about it earlier, but these flowers are Asclepias fascicularis (Narrow leaved Milkweed - see also one of its narrow leaves) which grows in the Livermore area. This is the most common milkweed in CA.
Thanks for clearing up the 'butterfly weed' mystery; so that appears to be one (I found four) of the common names for A. tuberosa.
A question for Beth: I thought that Alameda Creek is closer to Fremont (Sunol Regional Wilderness through Niles Canyon), or is there another Alameda Creek?

Hi . I have the orange " butterfly weed " in my yard and I know it is a milkweed plant . The purple flowering version is a close relative and the butterflys love it so I call it a Butterfly weed .
I am sure that this creek travels a long way and probably is the same creek that goes through Sunol . They named the street Alameda creek road , in Livermore , where the creek meanders through the fields and hills . It has water in it which is odd this time of year , so I believe it is being maintained by the city to uphold the variety of wildlife that depends on it . There are Crawdads , little fish , Dragonflies , Butterflies , Bees and bugs also a wide variety of shore birds including Egrets and the great blue Heron . I am pretty sure it is the same creek that runs through Sycamore grove park as well . I know that it's course has been changed by people many times over the years because of roads and houses , but I believe it is most natural through our protected parks who preserve it's natural beauty . I know there is a watershed / dam near Sunol and the park service can control water regulation . Thanks for your responses , sincerely Beth

for your description of the area where you saw the wasp. I think I now know what you're talking about - see this site of the Alameda Creek Alliance.