Arrowrock Res — storage

Storage now, against this reservoir's own record

215.00 Mm³

Stored, September 1986

100th

Percentile among 69 Seps on record

+279.2%

Change over the 12 months to 1986-09

Storage in September 1986 was higher than almost every other Sep on record, ranked against 69 other Seps held for this reservoir. It is the highest in the held record for this month.

No percentage departure from the Sep average is published here. No long-term average is held for this month, so there is nothing to compare with.

Storage, 1917–1986

Monthly mean storage

828 monthly values, 1917 to 1986

Monthly means of the publisher's daily record. Volume, not percentage of capacity — see below for why.

What the shaded periods are (0 events)

Not shown on this chart

The 1976 drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
The 2022 drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Yorkshire drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Cantareira water crisis, São Paulo — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Cape Town Day Zero — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
European drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.

View the data behind this chart

240 of 828 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 828 would be longer than this page and unusable with a screen reader.
Period	Arrowrock Res At Arrowrock Dam ID (Mm³)
October 1917	6.32
January 1918	132
May 1918	329
August 1918	148
December 1918	106
March 1919	94.2
July 1919	203
October 1919	12.2
February 1920	56.8
May 1920	255
September 1920	3.25
December 1920	39.6
April 1921	285
July 1921	314
October 1921	19.3
February 1922	145
May 1922	302
September 1922	16.1
December 1922	20.3
April 1923	143
July 1923	319
November 1923	26.2
February 1924	64.8
June 1924	85.6
September 1924	0
January 1925	53.6
April 1925	309
July 1925	305
November 1925	24.1
February 1926	58.2
June 1926	258
September 1926	9.19
January 1927	109
April 1927	222
August 1927	220
November 1927	86.8
March 1928	303
June 1928	343
September 1928	40.8
January 1929	81.8
April 1929	184
August 1929	116
November 1929	14.5
March 1930	84.7
June 1930	338
October 1930	18.4
January 1931	57.9
May 1931	194
August 1931	8.37
December 1931	23.5
March 1932	33.2
June 1932	345
October 1932	14.5
January 1933	48.9
May 1933	196
August 1933	139
December 1933	13.7
March 1934	165
July 1934	153
October 1934	4.96
February 1935	69.6
May 1935	339
September 1935	40.3
December 1935	19.6
March 1936	103
July 1936	248
October 1936	10.5
February 1937	84.9
May 1937	307
September 1937	24.8
December 1937	62.6
April 1938	285
July 1938	351
November 1938	95.4
February 1939	231
June 1939	348
September 1939	37.2
December 1939	26.8
April 1940	357
July 1940	259
November 1940	94.9
February 1941	192
June 1941	366
September 1941	72.7
January 1942	212
April 1942	351
August 1942	177
November 1942	53.4
March 1943	146
June 1943	362
September 1943	132
January 1944	192
April 1944	327
August 1944	144
November 1944	57.9
March 1945	185
June 1945	370
October 1945	23.8
January 1946	118
May 1946	332
August 1946	244
November 1946	138
March 1947	315
June 1947	366
October 1947	65.8
January 1948	152
May 1948	364
August 1948	222
December 1948	92.4
March 1949	230
July 1949	277
October 1949	47.7
February 1950	130
May 1950	184
August 1950	264
December 1950	84.3
March 1951	244
July 1951	332
October 1951	81.1
February 1952	247
May 1952	108
September 1952	61.1
December 1952	165
April 1953	255
July 1953	340
November 1953	69.8
February 1954	322
May 1954	279
September 1954	57.3
December 1954	112
April 1955	287
July 1955	90.8
November 1955	70.4
February 1956	313
June 1956	353
September 1956	23.6
January 1957	293
April 1957	337
August 1957	110
November 1957	83.3
February 1958	286
June 1958	352
September 1958	50.9
January 1959	258
April 1959	343
August 1959	44.4
November 1959	152
March 1960	290
June 1960	347
October 1960	7.13
January 1961	194
April 1961	335
August 1961	5.03
November 1961	101
March 1962	301
June 1962	282
October 1962	27.1
January 1963	289
May 1963	349
August 1963	70.8
December 1963	244
March 1964	338
July 1964	294
October 1964	53.8
January 1965	343
May 1965	326
August 1965	227
December 1965	323
March 1966	341
July 1966	104
October 1966	53.6
February 1967	192
May 1967	251
September 1967	5.28
December 1967	174
April 1968	319
July 1968	23.5
October 1968	25.7
February 1969	314
May 1969	348
September 1969	6.96
December 1969	158
April 1970	345
July 1970	326
November 1970	148
February 1971	344
June 1971	342
September 1971	51.5
January 1972	341
April 1972	217
July 1972	312
November 1972	141
February 1973	342
June 1973	273
September 1973	8.01
January 1974	280
April 1974	317
August 1974	181
November 1974	132
March 1975	344
June 1975	142
October 1975	92.3
January 1976	343
April 1976	319
August 1976	107
November 1976	109
March 1977	180
June 1977	11.3
October 1977	25.0
January 1978	195
May 1978	169
August 1978	109
December 1978	196
March 1979	345
June 1979	257
October 1979	64.0
January 1980	233
May 1980	351
August 1980	172
December 1980	149
March 1981	353
July 1981	230
October 1981	52.5
February 1982	263
May 1982	238
September 1982	126
December 1982	132
March 1983	152
July 1983	332
October 1983	66.8
February 1984	89.9
May 1984	245
September 1984	140
December 1984	169
April 1985	273
July 1985	222
November 1985	189
February 1986	346
June 1986	343
September 1986	215

Why this page shows a volume and not a percentage

Percentage full is a division, and the denominator is missing

“Percentage full” is the figure a drought is discussed in, and it is stored volume divided by capacity. Capacity is published on several different bases — gross, usable, live, and the figure at a particular spillway or licence condition — and dividing a stored volume by the wrong one produces a percentage confidently wrong by the ratio between them, at exactly the moment the number matters most.

Hydrionis holds storage measurements for this reservoir and does not hold a compatible authoritative capacity figure for it, so percentage full is not calculated. The volume above is what was actually measured. Where you need a percentage, take the publisher’s own capacity on its own stated basis and divide it yourself — the source is linked below, and knowing which basis you used is the whole point.

Rate and persistence

+13.19 Mm³

Mean change per month over 12 months

2 of 24

Recent months below the 20th percentile for their own month

Rates are computed only across an unbroken run of months, so a rate is never divided by a period in which some of the change was not observed. Percentile counts compare each month with the same calendar month in every other year held.

Location

Arrowrock Res At Arrowrock Dam ID

Gauge location

About this data

Everything below is a property of the data rather than a rating of it. There is deliberately no single confidence score: how much a gap matters depends on what you are about to do with the series, and only you know that.

Source	United States Geological Survey — National Water Information System · US Government work — public domain
How the values came to exist	Measured
Period held	1917–1986
Completeness	98.6% of the periods in that span are held. Gaps are drawn as breaks and never interpolated.
Known gaps	Storage is a managed quantity. A falling line can be a rule curve, a scheduled release or a transfer to another system rather than a shortage of water, and nothing here distinguishes them. No capacity figure on a stated basis is held for this reservoir, so no percentage of capacity is computed.
Continuity	99% of the months between 1917 and 1986 are held. Monthly means of the publisher's own daily record.
Last read from the publisher	2026-10-01
Redistribution	Permitted by the upstream licences
Commercial reuse	Permitted by the upstream licences. No Hydrionis access tier can widen this.
Methodology	How this was produced

Use this data

Machine-readable JSON /api/v1/observations/by-subject/reservoir-usgs-13194000.json
How to cite this, and who you must credit
How these figures were produced
Request this at a different scale, format or geography

No bulk download is offered here, which is a licence position rather than a product one: at least one contributing publisher does not permit redistribution. The values can be read on the page, cited, and pointed to at source.

Attribution and citation

Suggested citation for Hydrionis

Hydrionis (2026), Arrowrock Res At Arrowrock Dam ID reservoir records, accessed 2026-10-05, hydrionis.com/reservoirs/reservoir-usgs-13194000/

Cite Hydrionis where you rely on the compilation, the normalisation, the eligibility judgements or a chart. Where you use an unmodified published value, the publisher’s attribution is the one that is required; crediting Hydrionis as well is welcome and is not something Hydrionis can require of you.

Recommended, not required. Hydrionis does not claim rights over unmodified public data it relayed, and doing arithmetic on somebody else’s measurements does not make them Hydrionis’s. Cite the version and the access date: the Arrowrock Res At Arrowrock Dam ID record changes as publishers release and as methods are revised, and a citation without both cannot be checked later. Full guidance is on cite and reuse.

None of the contributing licences requires attribution. Crediting the publishers is still good practice and they are listed above.

Where this comes from

828 monthly observations 1917–1986 Mm³

Spatially aggregatedHigh confidenceSeptember 1986

Source: USGS — National Water Information System, 13194000. Licence: Open.

Method: Monthly mean of daily reservoir storage published by the USGS in acre-feet and converted to millions of cubic metres. Months with fewer than 20 days of record are not published rather than averaged from a partial month.

Reservoir storage is managed as well as observed: a drawdown may reflect an operating rule, flood-control rule curve or scheduled release rather than a shortage. Compare a month against the same month in other years, and do not read a fall as a supply signal without knowing the operating regime.

Retrieved 1 October 2026.

Monthly mean of daily reservoir storage published by the USGS in acre-feet and converted to millions of cubic metres. Months with fewer than 20 days of record are not published rather than averaged from a partial month.