While tools like rtl_power and rx_power now exist for creating long term averaged waterfalls for many SDR’s, another option is to use a screenshot grabber to grab screenshots of the waterfall every few seconds on an SDR program like HDSDR.
This is what the admin of the coolsdrstuff.blogspot.com blog has done. The author used the program Chronolapse which was set to take a screenshot every 60 seconds. The waterfall in HDSDR was then set to a speed so that the waterfall would complete one cycle every 60 seconds. Then after collecting images all night he used Irfanview to bulk resize all the images to be 1 pixel high. Finally he then combined all the 1 pixel high images into a nice waterfall image.
The waterfall speed in HDSDR can also be set to a very slow update speed, but the problem with this as noted by the author is that this does not average the data, meaning that data in between waterfall updates is lost.