Are these birch or aspen trees?
Turns out this is harder than it seems from just the picture...

The three candidates I think one could most argue for are:


Quaking Aspen (Populus tremuloides)
Paper Birch (Betula papyrifera)
Grey Birch (Betula populifolia)


Here is a picture of Populus tremuloides from the USDA:



Notice the smoothness of the above Aspen bark (identical to that of the picture in the OP's question). Now compare it to the peely nature of typical birch bark (here pictured B. papyrifera or paper birch):



Though not all birch species have peely bark: (my emphasis below) 


Grey birch (Betula populifolia) typically remains fairly smooth. From Viginia Tech's tree guide: 


  Bark: Reddish brown with numerous lighter lenticels on very young stems, later turning gray to white and very chalky; remains smooth and generally does not peel.

B. pendula (European white birch) bark is also described as being smooth and only somewhat peely. From Virginia Tech:


  Bark: Reddish brown with light lenticels when very young, later turning white, generally smooth but does peel a bit; the base of the tree develops thick bark which splits into narrow, vertical furrows which are nearly black.



However, the branches in the OP's picture are not drooping as is also common for B. pendula (AKA the European weeping birch). So this is not our tree!

Also, some sources describe paper birch as simply being smooth (vs. emphasizing the peeliness). See here for an example.


Here are images of European white birch (left) and gray birch (right) :





But Gray birch remains smooth, even at the base of the tree (though see here)...

Pay attention to the bottom of the trees in the OP's picture. The bark at the base of the trees is becoming furrowed:

Zoomed in OP picture: 



This base furrowing is a characteristic of P. tremuloides:


According to the Virginia Tech Tree Guide (my emphasis):


  Bark: At first smooth, creamy yellowish-white to very light green; later developing thick furrows and becoming dark, especially near the base.

And according to The Sibley Guide to Trees  (my emphasis):


  Bark color from white to greenish, grayish, or bronze. Bark smooth, pale with dark scars that persist for many years; base of trunk becomes coarsely ridged grey on old trunks.

And from the NDSU Agriculture webpage (my emphasis):


  Bark Color - Varies from chalky white to olive-gray, and becomes rough furrowed on the lower trunk of mature trees.

[Note: I give all 3 sources to demonstrate the inconsistency in description of P. tremuloides bark color].


Here is a close-up of furrowed aspen bark from the USDA website (yes, this is still aspen):



However, this furrowing can also occur in paper birch. 


From V tech (my emphasis):


  Bark: Reddish brown with light lenticels on very young stems; later turning chalky to creamy white, peeling in horizontal papery strips; brown to black and may be furrowed at base; orange inner bark.

From Field Guide to Trees of North America (my emphasis):


  Bark white papery; older trees have rough black bark near base.



I will comment that I have never seen paper birch with furrowed bark at the base. Further, I was unable to find visual evidence from any reputable sources by searching on Google. 



So what about tree shape and form?

Again, according to The Sibley Guide to Trees:


  [P. tremuloides has a] single main trunk with relatively stout , jagged twigs (unlike more slender and graceful twigs of birches). 
  
  [B. papyrifera] branches begin lower on trunk than in aspens; more graceful, ascending, and regular with slender straight twigs. Mature tree has oval habit, few strong branches, clumps of fine twigs in the crown.
  
  [B. populifolia has] typically multiple leaning, crooked trunks and narrow crowns. Bark of mature trunks grayish white, nonpeeling. 


Citing a different source (Field Guide to Trees of North America) to address different language to differentiate the species:


  [P. tremuloides:] Small to medium-sized tree. slender trunk; pyrimidal to round crown of spreading branches with sparse foliage. 
  
  [B. papyrifera:] Medium-sized tree. One to several slender, straight trunks; narrow to broad crown; branches upraised in youth, often becoming horizontal or drooping in age
  
  [B. populifolia:] Small tree. sometimes shrubby; One to several trunks; narrow, conical crown. 


The smaller trees in the front left of the OP's picture demonstrate a more birch-like architecture and have branches growing along the stem as the description for B. papyrifera describes. These qualities are reduced or eliminated in the larger trees in the OP's picture.

For visual clarification, the typical pyrimidal crown shape of P. tremuloides is shown below (from Oregon State Department of Horticulture). The trees in this image appear more slender (pyrimidal) than the trees in the OP's image, but aspen trees can also have "round" crowns as stated above. 



The following photo from Michigan State University Extension's Tree ID Key attempts to demonstrate a difference between paper birch and aspen crown coloration:  



The crown branches/twigs visible in the OP's picture seem to be lighter colored, and so this line of evidence points to aspen.



Ok, so what about size? According to Field Guide to Trees of North America the typical sizes of the species under question are fairly similar, but with gray birch being smaller than the other two species:


Quaking Aspen (Populus tremuloides) = 30-70' tall, 1-1.5' diameter
Paper Birch (Betula papyrifera) = 50-70' tall, 1-2' diameter
Grey Birch (Betula populifolia) = 20-30' tall, 0.5-1' diameter


The relative sizes of the trees in the OP's picture don't seem to preclude any of these species.

Both birches and aspens develop yellow leaves in autumn, so this characteristic additionally is not useful for differentiating the two genera (or 3 species). 



Though @fileunderwater is correct in stating that birches have horizontal lenticels that are helpful in identifying birch trees, the OP's image is far too zoomed out to make out any lenticels (even when zooming into the OP's picture). Those black spots are not lenticels; they are scars (generally from lost limbs). Regardless, other species (including some aspen species) also develop lenticels.

Here is a close-up of lenticels. The little slits are birch lenticels (here from B. papyrifera):

  



Names on files and webpages are notoriously incorrect so using that as a line of evidence is dangerous. I saw a number of misidentified species in pictures while I was searching myself.



So thus far it seems as though aspen is a pretty good choice, but paper birch could also be potentially correct. The distance of the picture makes this distinction difficult, but I will point to the two lines of evidence that have convinced me that some of the trees are in fact paper birches. 


The girth and shape of the larger trees are more consistent with the growth form of birches vs. the usually slender, single-stemmed aspen.
In the OP's picture, you cannot see peeling or lenticels, but this might be due simply to the distance from the trees. However, you can see a brownish/orangish coloration on a number of the trees, which is consistent with numerous descriptions of paper birch trees that have already started peeling revealing brown/orange bark.  




Conclusion = Hard to tell, and likely actually a mix of Paper Birch (Betula papyrifera) and Quaking Aspen (Populus tremuloides). I think you're looking at taller aspen and mostly wider paper birch.