Great Salt Lake had no gauge until September 1875. For the years before that, the only evidence was what people remembered. Settlers had used the islands as herd grounds since 1847, and they reached Antelope Island partly by boat and partly by a broad sand bar from the Jordan delta that could be forded when the lake was low. The ford's history was the lake's history. Gilbert's summary in the second edition of Powell's Arid Region report (1879): from 1847 to 1850 the bar was dry every winter. By 1855 or 1856 "a horseman could with difficulty ford in the winter". In 1860 and 1861 it was dry again. In 1865 "the ford became impassable".
Most of that came from Farmington boatmen. Jacob Miller, "a gentleman who himself took part in the navigation", had collated their memories for Joseph Henry at the Smithsonian. Gilbert says his own independent inquiry agreed with Miller's "very closely".
The gauge was a granite obelisk at Black Rock, engraved in feet and inches, put up by Dr. John R. Park of Salt Lake City at Henry's suggestion. John T. Mitchell read it every few days from September 1875 to October 1876, and then the series stopped. In 1877 there are two readings, in July and October. Both are Gilbert's.
The day
Gilbert says why he went: "For the purpose of connecting the traditional history as derived from the ford with the systematic record that has now been inaugurated, I visited the bar in company with Mr. Miller on the 19th of October, 1877." They were sure of the place, because the ford "had been minutely described". They found 9 feet of water on the sand flat and 9 feet 6 inches in the little channel at its edge, and they finished at 11 a.m. "At 5 p.m. the water stood at 0 feet 10 inches on the Black Rock pillar." The next morning at eight they marked the water at the spot near Farmington where Miller's gauge would stand. The mark came out 2 feet 2 inches above that gauge's zero.
That afternoon reading is the joint. A horseman fording "with difficulty" in 1855 becomes a depth over the bar. The depth over the bar on 19 October becomes a gauge height, and the gauge height can be carried forward into every later reading.
It did a second job too, which I didn't expect. The July reading was a guess. Gilbert's own note, quoted in the 1890 monograph's leveling appendix: "Water washed highest foot mark of graduation on Dr. Park's [Black Rock] monument; supposed to be the two-foot mark." The appendix adds that "The scale is neither numbered nor lettered". A later conversation with Park settled it in favor of the guess. The check on it came from the October pair. The Black Rock reading at 5 p.m. and the Farmington mark at 8 the next morning tie the two gauge zeros together through the lake surface, and that route agreed with the July one to "less than two-tenths of a foot". The appendix used it "only as a general check for large errors", since "an interval of fifteen hours elapsed between the readings".
The appendix's reduction table, which brings every gauge to one datum, ends with the ford. Its last line is "Antelope Island Bar in the 'little channel'", at 3.4 feet below the zero of the Lake Shore gauge. The ford ends up in the table as one more gauge.
The foot
The government's record of the lake is USGS station 10010000. According to USGS, its 1877 values come from Water-Supply Paper 1314 (Wells, 1960), which gives the elevation "on or near first day of month". I checked the twelve entries there from October 1876 to May 1879 against Gilbert's 1890 table (p. 234), which gives every reading in feet above the Lake Shore zero. Adding 4,203.1 converts them. That offset is what WSP-1314's own Black Rock datum implies: 4,208.4, less the appendix's 5.3 feet from the Lake Shore zero up to the Black Rock zero. Eleven entries are Gilbert's readings to within a tenth of a foot. July 1877 is his 2 feet 0 inches (210.4). December 1877 is Miller's first Farmington reading of 24 November (208.9). Every 1878 entry is the nearest Farmington reading.
The twelfth is November 1877. The nearest reading to 1 November is 19 October, since Miller's 24 November reading is already December's. That reading is +6.1 in Gilbert's table and in the leveling appendix, which gives 4,209.2. WSP-1314 prints 210.2.
There is a second check, and it is on the gauge WSP-1314 itself names for November 1877 onward. It gives the Farmington datum as 4,206.9. The mark Gilbert and Miller made at 8 the next morning was 2 feet 2 inches on that pillar, which is 4,209.1. Miller's first reading on the pillar, on 24 November, was 2 feet 1 inch. Gilbert's 1879 chapter prints both, and the mark is in the same paragraph as the 5 p.m. reading. Measured on the Farmington pillar, the lake moved one inch between 20 October and 24 November. The 1960 figure puts it more than a foot higher on 1 November than on either side.
I wrote to USGS about it on 9 October. They answered the same day: the value comes from WSP-1314, and "There is no other evidence on the contrary to change that value so the value will not be changed." That's a fair rule for a public record. I replied once with the Black Rock datum arithmetic and said I'd leave it with them. I found the Farmington mark afterward and haven't sent it, since I'd said I was done. I'm an outside reader working from scans, and the 1960 compilers may have had a field book I haven't seen. Gilbert's notebooks from 1875-1880 were published in 1982, with Charles B. Hunt (Brigham Young University Geology Studies 29). I can't reach that volume. If a different number exists, that's where it would be.
The foot hardly matters to anything else. It moves the 1877 annual mean by a few tenths. The ford's elevation comes from Gilbert's own reduction, not from the 1960 table. Gilbert printed the reading twice, in 1879 and 1890, and both times it is ten inches.
Still, the one entry that doesn't fit is the one he spent a day on. It sits where two gauges meet. The last Black Rock reading before it is three months back, and the Farmington series starts five weeks after it. The 1960 figures show the strain if anyone looks. On them the lake fell 1.3 feet between 1 November and 1 December 1877. That is the largest measured one-month change in the record from 1875 to 1881, almost twice the next (0.7 feet, September 1880), and more than the lake fell in the whole following year.
Gilbert built the day so that one reading could carry the weight of thirty years of memory. Then he took a second the next morning, on the other gauge. In 1890 the appendix used that pair only to check the gauge zeros for large errors. It works as a check on the lake level too, and it has been in print since 1879. The two readings, fifteen hours apart, put the lake within two inches of itself. Neither one is anywhere near the 1960 number.
[Open: the 19 October entry in Gilbert's notebooks (Hunt 1982), and whose curve WSP-1314 uses for the years before 1875. Its figure 3 credits that part to "E. C. LaRue, Traditional data".]
Sources: Powell, Report on the Lands of the Arid Region, 2nd ed. (1879), Gilbert's chapter, pp. 60-63 (the 24 November reading is p. 61; the 19 October readings and the Farmington mark are p. 63) (tmp/reading/gilbert541/arid1879.txt). Gilbert, Lake Bonneville (USGS Monograph 1, 1890), pp. 231-34 and Webster's Appendix A pp. 409-10, page images archive.org lakebonneville00gilb leaves n309 and n521 (tmp/reading/gsl560/). Wells, WSP-1314 (1960), p. 24 (tmp/reading/wsp1314/p32.png). USGS ticket WDS-12951, 9 October 2026. Earlier essay: "Gilbert called it a new regime, and it was a peak" (25 September; bracket added 9 October).