Arrowrock Reservoir — storage

Storage now, against this reservoir's own record

68.95 Mm³

Stored, August 2026

29th

Percentile among 109 Augs on record

-35.5%

Against the Aug 1991–2020 average

-19.5%

Change over the 12 months to 2026-08

Storage in August 2026 was lower than most Augs on record, ranked against 109 other Augs held for this reservoir.

Storage, 1917–2026

Monthly mean storage

1,307 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 1,307 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 1,307 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
December 1930	24.4
June 1931	155
November 1931	9.97
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
October 1937	3.81
April 1938	285
September 1938	103
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
September 1943	132
March 1944	247
August 1944	143
February 1945	154
July 1945	325
January 1946	118
June 1946	359
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
July 1950	351
January 1951	165
June 1951	343
December 1951	308
May 1952	108
November 1952	96.3
April 1953	255
October 1953	27.3
March 1954	287
September 1954	57.6
February 1955	233
August 1955	124
January 1956	334
June 1956	353
December 1956	249
May 1957	281
November 1957	83.4
April 1958	159
October 1958	67.8
March 1959	338
September 1959	12.6
February 1960	275
August 1960	34.7
January 1961	194
July 1961	13.7
December 1961	191
June 1962	282
November 1962	125
April 1963	353
October 1963	85.1
March 1964	338
September 1964	21.0
February 1965	344
August 1965	224
January 1966	332
July 1966	101
December 1966	135
June 1967	320
November 1967	99.9
May 1968	128
October 1968	27.0
March 1969	132
September 1969	6.76
February 1970	342
August 1970	136
January 1971	344
July 1971	345
December 1971	314
June 1972	347
November 1972	143
May 1973	320
October 1973	49.0
April 1974	317
September 1974	50.2
March 1975	344
August 1975	166
January 1976	343
July 1976	253
December 1976	158
June 1977	11.3
November 1977	67.0
May 1978	169
October 1978	95.0
April 1979	344
September 1979	49.6
March 1980	345
August 1980	172
February 1981	301
July 1981	226
December 1981	188
June 1982	338
November 1982	199
May 1983	221
October 1983	66.8
April 1984	211
September 1984	140
March 1985	295
August 1985	72.6
February 1986	346
July 1986	276
January 1987	268
June 1987	127
December 1987	116
May 1988	91.8
October 1988	16.7
April 1989	310
September 1989	7.26
March 1990	195
August 1990	38.4
February 1991	277
July 1991	23.6
January 1992	138
June 1992	18.9
December 1992	109
May 1993	334
November 1993	200
April 1994	220
September 1994	16.1
March 1995	289
August 1995	213
February 1996	278
July 1996	335
January 1997	319
June 1997	330
December 1997	201
May 1998	348
November 1998	137
April 1999	126
October 1999	81.9
March 2000	306
September 2000	54.3
February 2001	172
July 2001	99.8
January 2002	233
June 2002	248
December 2002	76.3
May 2003	228
November 2003	1.75
April 2004	231
October 2004	67.9
March 2005	191
September 2005	55.4
February 2006	211
August 2006	135
January 2007	265
June 2007	237
December 2007	189
May 2008	306
November 2008	141
April 2009	308
October 2009	98.0
March 2010	283
September 2010	135
February 2011	275
August 2011	211
January 2012	266
July 2012	248
December 2012	191
June 2013	170
November 2013	160
April 2014	299
October 2014	106
March 2015	317
September 2015	70.0
February 2016	267
August 2016	73.4
January 2017	235
July 2017	283
December 2017	304
June 2018	277
November 2018	132
May 2019	245
October 2019	101
March 2020	290
September 2020	83.0
February 2021	268
August 2021	72.7
January 2022	191
July 2022	208
December 2022	134
June 2023	307
November 2023	143
May 2024	267
October 2024	74.2
April 2025	129
September 2025	91.2
March 2026	308
August 2026	69.0

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	69.0
Sep	65.9	91.2
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

551 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 551 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 551 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
1986.0833333333333	980
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	962
1988.25	964
1988.4166666666667	944
1988.5833333333333	920
1988.8333333333333	944
1989	960
1989.1666666666667	969
1989.4166666666667	967
1989.5833333333333	927
1989.75	929
1989.9166666666667	950
1990.1666666666667	965
1990.3333333333333	950
1990.5	954
1990.75	948
1990.9166666666667	963
1991.0833333333333	974
1991.3333333333333	935
1991.5	934
1991.6666666666667	926
1991.8333333333333	947
1992.0833333333333	961
1992.25	959
1992.4166666666667	932
1992.6666666666667	937
1992.8333333333333	950
1993	958
1993.25	976
1993.4166666666667	980
1993.5833333333333	959
1993.75	958
1994	974
1994.1666666666667	973
1994.3333333333333	954
1994.5833333333333	931
1994.75	933
1994.9166666666667	950
1995.1666666666667	975
1995.3333333333333	974
1995.5	979
1995.6666666666667	946
1995.9166666666667	971
1996.0833333333333	974
1996.25	965
1996.5	979
1996.6666666666667	944
1996.8333333333333	961
1997.0833333333333	963
1997.25	950
1997.4166666666667	978
1997.6666666666667	943
1997.8333333333333	958
1998	971
1998.1666666666667	976
1998.4166666666667	980
1998.5833333333333	963
1998.75	952
1999	971
1999.1666666666667	964
1999.3333333333333	962
1999.5833333333333	960
1999.75	949
1999.9166666666667	960
2000.0833333333333	973
2000.3333333333333	979
2000.5	971
2000.6666666666667	943
2000.9166666666667	954
2001.0833333333333	965
2001.25	971
2001.5	954
2001.6666666666667	950
2001.8333333333333	960
2002	971
2002.25	979
2002.4166666666667	973
2002.5833333333333	942
2002.8333333333333	952
2003	958
2003.1666666666667	973
2003.4166666666667	979
2003.5833333333333	939
2003.75	923
2003.9166666666667	922
2004.1666666666667	947
2004.3333333333333	969
2004.5	965
2004.75	949
2004.9166666666667	955
2005.0833333333333	964
2005.3333333333333	966
2005.5	962
2005.6666666666667	947
2005.8333333333333	954
2006.0833333333333	969
2006.25	965
2006.4166666666667	979
2006.6666666666667	949
2006.8333333333333	964
2007	974
2007.25	978
2007.4166666666667	971
2007.5833333333333	947
2007.75	954
2008	971
2008.1666666666667	977
2008.3333333333333	978
2008.5833333333333	958
2008.75	953
2008.9166666666667	967
2009.1666666666667	977
2009.3333333333333	976
2009.5	973
2009.6666666666667	950
2009.9166666666667	966
2010.0833333333333	974
2010.25	976
2010.5	975
2010.6666666666667	960
2010.8333333333333	959
2011.0833333333333	975
2011.25	970
2011.4166666666667	973
2011.6666666666667	957
2011.8333333333333	965
2012	974
2012.1666666666667	977
2012.4166666666667	979
2012.5833333333333	959
2012.75	956
2013	971
2013.1666666666667	975
2013.3333333333333	968
2013.5833333333333	949
2013.75	956
2013.9166666666667	969
2014.0833333333333	977
2014.3333333333333	978
2014.5	966
2014.6666666666667	954
2014.9166666666667	967
2015.0833333333333	978
2015.25	979
2015.5	963
2015.6666666666667	950
2015.8333333333333	959
2016	969
2016.25	977
2016.4166666666667	975
2016.5833333333333	950
2016.8333333333333	962
2017	971
2017.1666666666667	964
2017.4166666666667	979
2017.5833333333333	966
2017.75	966
2017.9166666666667	978
2018.1666666666667	978
2018.3333333333333	979
2018.5	967
2018.75	955
2018.9166666666667	964
2019.0833333333333	971
2019.3333333333333	972
2019.5	971
2019.6666666666667	947
2019.8333333333333	962
2020.0833333333333	977
2020.25	972
2020.4166666666667	977
2020.6666666666667	952
2020.8333333333333	961
2021	970
2021.25	973
2021.4166666666667	967
2021.5833333333333	950
2021.75	950
2022	967
2022.1666666666667	976
2022.3333333333333	976
2022.5833333333333	951
2022.75	950
2022.9166666666667	960
2023.1666666666667	972
2023.3333333333333	968
2023.5	971
2023.6666666666667	958
2023.9166666666667	966
2024.0833333333333	975
2024.25	975
2024.5	967
2024.6666666666667	963
2024.8333333333333	952
2025.0833333333333	969
2025.25	959
2025.4166666666667	977
2025.5833333333333	952
2025.8333333333333	961
2026	977
2026.1666666666667	978
2026.4166666666667	972
2026.5833333333333	950

Rate and persistence

-1.39 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	99.0% 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	2026-09-01
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 2026-09-02, 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

1,307 monthly observations 1917–2026 Mm³

Spatially aggregatedHigh confidenceAugust 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 September 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.