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Found on flowers of Darmera peltata growing at the edge of a creek in a montane mixed-coniferous forest of the central Sierra Nevada.

Using the USDA "idtools.org" interactive key for sawflies, selecting just two antennal characters...namely:

1) "antennae clavate...only slightly expanded towards the apex" (i.e. not "remarkably clubbed"); and
2) "number of antennal segments 17-22" (cf. reference image here and the 2nd image in this series)
...narrows things down to one of Calameuta, Cephus, or Trachelus. Remarkably, trying most of the other characters in that interactive key would not seem to separate these three genera.

But according to the 3 preceding links, only 5 species in those genera are recorded from North America: Calameuta clavata, Calameuta middlekauffi, Cephus cinctus, Cephus pygmaeus and Trachelus tabidus. Note that Ca. middlekauffi was described relatively recently in Smith & Schiff(1)(2005)...and the 4 other earlier-named species were treated as members of either Tribe Cephini in Middlekauff(2)(1969); or simply as members of the genus Cephus in the older Ries(3)(1937)...see here.

I found it not entirely straightforward to work these photos through the keys in the above references...but by cross-referencing between the descriptions, figures, and keys in those papers I arrived at Cephus cinctus...based mainly on the ventral view of the distal portion of the abdomen in last photo of this series. In particular, the distal edge of the 8th sternite is seen there to be more-or-less straight...as specified in couplet 4 of the key in Ries(3)(1937) and shown in his Fig. 5.

However, the distal edge of the 8th sternite is notched in:

A) Calameuta clavata...see couplet 1) of the key on pg. 9 of Middlekauff(2)(1969); Fig. 9 in Ries(3)(1937); and the ventral shots below:
 
B) Calameuta middlekauffi...see the last sentence of the description of the male on pg. 865 of Smith & Schiff(1)(2005);
C) Cephus pygmaeus...per couplet 4) and Fig. 4 of in Ries(3)(1937). However, this seems contradicted by the key in Middlekauff(2)(1969)...where pygmaeus and cinctus both come out at the 2nd lead of couplet 1), which indicates no notch on the posterior edge of the 8th sternite! Nevertheless, I think pygmaeus can be eliminated here, since the keys in both Middlekauff and Ries indicate it has costa & stigma uniformly brown, whereas cinctus has costa yellow and stigma black...as seen in the photos of this series.
Based on Fig. 10 in Ries(3)(1937) it appears the 8th sternite is also somewhat notched in Trachelus tabidus. But regardless of that, T. tabidus has distinctive spine-filled cavities on sternites 7 & 8...which are not evident in my last photo here.

So, in summary, Cephus cinctus appears to be the ID here.