Arrowrock Reservoir — storage

Storage now, against this reservoir's own record

89.58 Mm³

Stored, September 2026

83rd

Percentile among 109 Seps on record

+35.9%

Against the Sep 1991–2020 average

-1.8%

Change over the 12 months to 2026-09

Storage in September 2026 was higher than most Seps on record, ranked against 109 other Seps held for this reservoir.

Storage, 1917–2026

Monthly mean storage

1,308 monthly values, 1917 to 2026

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 1,308 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 1,308 would be longer than this page and unusable with a screen reader.
Period	Arrowrock Reservoir (Mm³)
October 1917	6.32
March 1918	176
September 1918	50.2
February 1919	98.0
August 1919	50.9
January 1920	60.8
July 1920	265
December 1920	39.6
June 1921	345
November 1921	26.0
May 1922	302
October 1922	2.85
April 1923	143
September 1923	54.0
March 1924	45.1
August 1924	1.92
January 1925	53.6
July 1925	305
December 1925	16.1
June 1926	258
November 1926	18.0
May 1927	284
October 1927	22.4
April 1928	314
September 1928	40.8
March 1929	131
August 1929	116
February 1930	69.0
July 1930	237
January 1931	57.9
June 1931	155
December 1931	23.5
May 1932	337
October 1932	14.5
April 1933	72.8
September 1933	34.5
March 1934	165
August 1934	56.6
February 1935	69.6
July 1935	276
January 1936	41.3
June 1936	346
December 1936	51.2
May 1937	307
November 1937	25.3
April 1938	285
October 1938	40.2
March 1939	255
August 1939	119
February 1940	117
July 1940	259
January 1941	149
June 1941	366
December 1941	130
May 1942	345
November 1942	53.9
April 1943	268
October 1943	59.4
March 1944	247
September 1944	34.2
February 1945	154
August 1945	178
January 1946	118
July 1946	351
December 1946	182
May 1947	368
November 1947	72.9
April 1948	280
October 1948	62.0
March 1949	230
September 1949	84.8
February 1950	130
August 1950	264
January 1951	165
July 1951	332
December 1951	308
June 1952	330
November 1952	96.3
May 1953	297
October 1953	27.3
March 1954	287
September 1954	57.6
February 1955	233
August 1955	124
January 1956	334
July 1956	310
December 1956	249
June 1957	345
November 1957	83.4
May 1958	218
October 1958	67.8
April 1959	343
September 1959	12.6
March 1960	291
August 1960	34.7
February 1961	276
July 1961	13.7
December 1961	191
June 1962	282
November 1962	125
May 1963	349
October 1963	85.1
April 1964	344
September 1964	21.0
March 1965	218
August 1965	224
February 1966	328
July 1966	101
January 1967	150
June 1967	320
December 1967	175
May 1968	128
October 1968	27.0
April 1969	231
September 1969	6.76
March 1970	343
August 1970	136
February 1971	344
July 1971	345
January 1972	341
June 1972	347
December 1972	262
May 1973	320
November 1973	103
April 1974	317
October 1974	101
March 1975	344
September 1975	50.0
February 1976	342
July 1976	253
January 1977	214
June 1977	11.3
December 1977	139
May 1978	169
November 1978	144
April 1979	344
October 1979	64.0
March 1980	345
September 1980	150
February 1981	301
August 1981	54.9
January 1982	266
July 1982	346
December 1982	132
May 1983	221
November 1983	193
April 1984	211
October 1984	184
March 1985	295
September 1985	56.7
February 1986	346
August 1986	210
January 1987	268
July 1987	54.7
December 1987	116
June 1988	62.1
November 1988	58.9
May 1989	247
October 1989	14.5
April 1990	186
September 1990	38.5
February 1991	277
August 1991	18.0
January 1992	138
July 1992	12.3
December 1992	109
June 1993	346
November 1993	200
May 1994	116
October 1994	22.7
April 1995	325
September 1995	68.0
March 1996	180
August 1996	175
February 1997	183
July 1997	327
December 1997	201
June 1998	355
November 1998	137
May 1999	174
October 1999	81.9
April 2000	327
September 2000	54.3
March 2001	202
August 2001	52.7
February 2002	271
July 2002	110
January 2003	123
June 2003	320
December 2003	1.73
May 2004	216
November 2004	89.6
April 2005	205
September 2005	55.4
March 2006	93.8
August 2006	135
February 2007	300
July 2007	110
January 2008	233
June 2008	329
December 2008	198
May 2009	290
November 2009	133
April 2010	287
October 2010	132
March 2011	275
September 2011	113
February 2012	309
July 2012	248
January 2013	235
June 2013	170
December 2013	211
May 2014	303
November 2014	141
April 2015	321
October 2015	94.3
March 2016	282
September 2016	73.9
February 2017	288
August 2017	184
January 2018	311
July 2018	192
December 2018	167
June 2019	299
November 2019	155
April 2020	238
October 2020	109
March 2021	294
September 2021	58.1
February 2022	236
August 2022	77.2
January 2023	179
July 2023	234
December 2023	185
June 2024	301
November 2024	84.9
May 2025	220
October 2025	101
April 2026	323
September 2026	89.6

The reservoir's year

Seasonal storage profile

Each calendar month's reference average, against the most recent year

1991–2020 average
Most recent 12 months

Twelve separate averages, one per calendar month. A January average describes Januaries only. Months are plotted 1–12.

View the data behind this chart

Underlying data
Period	1991–2020 average (Mm³)	Most recent 12 months (Mm³)
Jan	218	293
Feb	249	302
Mar	242	308
Apr	249	323
May	245	303
Jun	266	248
Jul	210	152
Aug	107	69.3
Sep	65.9	89.6
Oct	91.3	101
Nov	129	141
Dec	173	199

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.

Water level

Monthly mean water surface elevation

552 monthly values

An elevation above a datum, not a depth of water and not a distance below the crest. Levels at different reservoirs are measured against different datums and must not be read against one another; this line is comparable only with its own history. Elevation and volume are not proportional either — a metre near the bottom of a reservoir holds far less water than a metre near the top, because the surface area grows with height.

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 552 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 552 would be longer than this page and unusable with a screen reader.
Period	Arrowrock Reservoir (m)
1980.75	949
1980.9166666666667	959
1981.1666666666667	980
1981.3333333333333	979
1981.5	968
1981.75	943
1981.9166666666667	964
1982.0833333333333	972
1982.25	959
1982.5	980
1982.6666666666667	956
1982.8333333333333	965
1983.0833333333333	953
1983.25	959
1983.4166666666667	980
1983.6666666666667	943
1983.8333333333333	964
1984	957
1984.1666666666667	955
1984.4166666666667	978
1984.5833333333333	961
1984.75	964
1985	963
1985.1666666666667	975
1985.3333333333333	975
1985.5833333333333	946
1985.75	954
1985.9166666666667	969
1986.1666666666667	977
1986.3333333333333	969
1986.5	973
1986.6666666666667	967
1986.9166666666667	967
1987.0833333333333	977
1987.25	966
1987.5	943
1987.6666666666667	927
1987.8333333333333	945
1988.0833333333333	964
1988.25	964
1988.4166666666667	944
1988.6666666666667	914
1988.8333333333333	944
1989	960
1989.1666666666667	969
1989.4166666666667	967
1989.5833333333333	927
1989.75	929
1990	955
1990.1666666666667	965
1990.3333333333333	950
1990.5833333333333	939
1990.75	948
1990.9166666666667	963
1991.0833333333333	974
1991.3333333333333	935
1991.5	934
1991.6666666666667	926
1991.9166666666667	954
1992.0833333333333	961
1992.25	959
1992.5	927
1992.6666666666667	937
1992.8333333333333	950
1993.0833333333333	962
1993.25	976
1993.4166666666667	980
1993.5833333333333	959
1993.8333333333333	966
1994	974
1994.1666666666667	973
1994.4166666666667	949
1994.5833333333333	931
1994.75	933
1995	958
1995.1666666666667	975
1995.3333333333333	974
1995.5833333333333	966
1995.75	951
1995.9166666666667	971
1996.0833333333333	974
1996.3333333333333	974
1996.5	979
1996.6666666666667	944
1996.9166666666667	972
1997.0833333333333	963
1997.25	950
1997.5	978
1997.6666666666667	943
1997.8333333333333	958
1998	971
1998.25	979
1998.4166666666667	980
1998.5833333333333	963
1998.8333333333333	958
1999	971
1999.1666666666667	964
1999.4166666666667	976
1999.5833333333333	960
1999.75	949
2000	966
2000.1666666666667	976
2000.3333333333333	979
2000.5	971
2000.75	945
2000.9166666666667	954
2001.0833333333333	965
2001.3333333333333	968
2001.5	954
2001.6666666666667	950
2001.9166666666667	966
2002.0833333333333	975
2002.25	979
2002.5	956
2002.6666666666667	947
2002.8333333333333	952
2003	958
2003.25	976
2003.4166666666667	979
2003.5833333333333	939
2003.8333333333333	922
2004	922
2004.1666666666667	947
2004.4166666666667	973
2004.5833333333333	946
2004.75	949
2004.9166666666667	955
2005.1666666666667	967
2005.3333333333333	966
2005.5	962
2005.75	949
2005.9166666666667	961
2006.0833333333333	969
2006.3333333333333	975
2006.5	976
2006.6666666666667	949
2006.9166666666667	970
2007.0833333333333	977
2007.25	978
2007.4166666666667	971
2007.6666666666667	949
2007.8333333333333	960
2008	971
2008.25	977
2008.4166666666667	980
2008.5833333333333	958
2008.8333333333333	961
2009	973
2009.1666666666667	977
2009.4166666666667	978
2009.5833333333333	963
2009.75	955
2009.9166666666667	966
2010.1666666666667	976
2010.3333333333333	972
2010.5	975
2010.75	960
2010.9166666666667	966
2011.0833333333333	975
2011.3333333333333	966
2011.5	977
2011.6666666666667	957
2011.8333333333333	965
2012.0833333333333	978
2012.25	975
2012.4166666666667	979
2012.6666666666667	951
2012.8333333333333	960
2013	971
2013.25	974
2013.4166666666667	964
2013.5833333333333	949
2013.8333333333333	963
2014	974
2014.1666666666667	977
2014.3333333333333	978
2014.5833333333333	951
2014.75	956
2014.9166666666667	967
2015.1666666666667	979
2015.3333333333333	976
2015.5	963
2015.75	954
2015.9166666666667	964
2016.0833333333333	974
2016.3333333333333	978
2016.5	964
2016.6666666666667	950
2016.8333333333333	962
2017.0833333333333	976
2017.25	964
2017.4166666666667	979
2017.6666666666667	959
2017.8333333333333	972
2018	978
2018.25	976
2018.4166666666667	975
2018.5833333333333	952
2018.75	955
2019	968
2019.1666666666667	966
2019.3333333333333	972
2019.5833333333333	954
2019.75	955
2019.9166666666667	968
2020.1666666666667	976
2020.3333333333333	972
2020.5	972
2020.75	956
2020.9166666666667	966
2021.0833333333333	974
2021.25	973
2021.5	952
2021.6666666666667	947
2021.8333333333333	956
2022.0833333333333	971
2022.25	977
2022.4166666666667	979
2022.6666666666667	953
2022.8333333333333	955
2023	965
2023.25	965
2023.4166666666667	978
2023.5833333333333	955
2023.75	957
2024	970
2024.1666666666667	974
2024.3333333333333	974
2024.5833333333333	968
2024.75	951
2024.9166666666667	958
2025.1666666666667	970
2025.3333333333333	970
2025.5	968
2025.6666666666667	953
2025.9166666666667	968
2026.0833333333333	978
2026.25	979
2026.5	962
2026.6666666666667	953

Rate and persistence

-0.14 Mm³

Mean drawdown 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 Reservoir

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 Bureau of Reclamation — Reservoir operations and Colorado River data · US Government work — public domain
How the values came to exist	Measured
Period held	1917–2026
Completeness	99.1% 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 2026 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-usbr-3642.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 Reservoir reservoir records, accessed 2026-10-05, hydrionis.com/reservoirs/reservoir-usbr-3642/

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 Reservoir 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

1,308 monthly observations 1917–2026 Mm³

Spatially aggregatedHigh confidenceSeptember 2026

Source: USBR — Reservoir operations and Colorado River data, RISE catalogue item 129217. Licence: Open.

Method: Monthly mean of the daily values published by the Bureau of Reclamation in acre-feet, converted to Mm³. Months with fewer than 20 days of record are not published rather than averaged from a partial month.

Stored as a volume and not as a share of capacity: the operator publishes what is in the reservoir but not what it holds when full, so a percentage cannot be computed without inventing the denominator.

Retrieved 1 October 2026.

Monthly mean of the daily values published by the Bureau of Reclamation in acre-feet, converted to Mm³. Months with fewer than 20 days of record are not published rather than averaged from a partial month.