Green Mountain Reservoir — storage

Storage now, against this reservoir's own record

33.23 Mm³

Stored, August 2026

0th

Percentile among 75 Augs on record

-80.4%

Against the Aug 1991–2020 average

-74.4%

Change over the 12 months to 2026-08

Storage in August 2026 was lower than almost every other Aug on record, ranked against 75 other Augs held for this reservoir. It is the lowest in the held record for this month.

Storage, 1945–2026

Monthly mean storage

904 monthly values, 1945 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 904 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 904 would be longer than this page and unusable with a screen reader.
Period	Green Mountain Reservoir (Mm³)
October 1945	173
February 1946	90.1
June 1946	141
September 1946	188
January 1947	124
May 1947	99.1
September 1947	189
December 1947	169
April 1948	64.4
August 1948	184
December 1948	133
April 1949	65.3
July 1949	189
November 1949	188
March 1950	111
July 1950	184
October 1950	154
February 1951	99.2
June 1951	85.5
October 1951	168
February 1952	128
May 1952	72.4
September 1952	180
January 1953	136
May 1953	70.3
August 1953	180
December 1953	127
April 1954	56.2
August 1954	103
December 1954	61.1
March 1955	53.2
July 1955	174
November 1955	124
March 1956	67.6
June 1956	149
October 1956	107
February 1957	78.3
June 1957	87.4
October 1957	181
January 1958	148
May 1958	71.3
September 1958	167
January 1959	106
April 1959	63.4
August 1959	186
December 1959	149
April 1960	76.4
August 1960	187
November 1960	140
March 1961	85.4
July 1961	177
November 1961	185
February 1962	125
June 1962	95.9
October 1962	123
May 1963	33.6
September 1963	128
December 1963	86.2
April 1964	75.9
August 1964	165
December 1964	116
March 1965	89.3
July 1965	183
November 1965	168
March 1966	96.3
July 1966	188
October 1966	123
February 1967	83.3
June 1967	105
October 1967	149
January 1968	115
May 1968	65.3
September 1968	182
January 1969	121
May 1969	103
August 1969	181
December 1969	136
April 1970	59.9
August 1970	183
November 1970	158
March 1971	87.2
July 1971	186
November 1971	150
March 1972	96.0
June 1972	141
October 1972	159
February 1973	110
June 1973	127
September 1973	176
January 1974	118
May 1974	73.1
September 1974	158
January 1975	108
April 1975	74.4
August 1975	187
December 1975	130
April 1976	77.5
July 1976	165
November 1976	136
March 1977	94.9
July 1977	142
November 1977	79.9
February 1978	66.3
June 1978	126
October 1978	145
February 1979	92.4
May 1979	90.3
September 1979	179
January 1980	115
May 1980	57.5
September 1980	161
December 1985	140
April 1986	55.2
August 1986	187
December 1986	164
March 1987	86.1
July 1987	177
November 1987	125
March 1988	85.1
July 1988	183
October 1988	111
February 1989	84.9
June 1989	153
October 1989	102
February 1990	81.0
May 1990	78.4
September 1990	110
January 1991	79.8
May 1991	83.9
August 1991	188
December 1991	130
April 1992	85.8
August 1992	171
December 1992	107
March 1993	92.3
July 1993	184
November 1993	158
March 1994	91.8
June 1994	136
October 1994	106
February 1995	82.7
June 1995	154
October 1995	184
February 1996	126
July 1996	186
November 1996	152
March 1997	94.6
June 1997	169
October 1997	167
February 1998	80.1
June 1998	72.5
December 1998	89.0
March 1999	82.8
July 1999	185
November 1999	130
March 2000	91.9
June 2000	178
October 2000	92.4
February 2001	64.5
June 2001	126
October 2001	108
January 2002	95.2
May 2002	86.0
September 2002	57.4
January 2003	50.2
April 2003	48.0
August 2003	177
December 2003	90.5
April 2004	89.2
August 2004	140
November 2004	105
March 2005	89.7
July 2005	188
November 2005	127
February 2006	99.0
June 2006	170
October 2006	137
February 2007	102
June 2007	175
September 2007	149
January 2008	107
May 2008	96.8
September 2008	161
December 2008	87.5
April 2009	82.1
August 2009	182
December 2009	100
April 2010	99.1
July 2010	188
November 2010	97.6
March 2011	85.6
July 2011	186
October 2011	147
February 2012	104
June 2012	135
October 2012	88.6
February 2013	75.5
May 2013	96.3
September 2013	129
January 2014	106
May 2014	99.0
August 2014	183
December 2014	101
April 2015	85.9
August 2015	184
December 2015	88.9
March 2016	78.9
July 2016	188
November 2016	95.1
March 2017	79.2
June 2017	150
October 2017	113
February 2018	85.7
June 2018	157
October 2018	83.1
January 2019	69.7
May 2019	90.0
September 2019	165
January 2020	90.0
April 2020	83.6
August 2020	167
December 2020	91.5
April 2021	76.1
August 2021	118
November 2021	84.9
March 2022	68.3
July 2022	159
November 2022	109
February 2023	96.1
June 2023	169
October 2023	120
February 2024	95.3
June 2024	164
September 2024	160
January 2025	86.1
May 2025	79.8
September 2025	113
December 2025	75.7
April 2026	67.2
August 2026	33.2

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	96.0	73.5
Feb	89.5	71.5
Mar	84.4	69.3
Apr	82.2	67.2
May	97.1	61.1
Jun	147	68.8
Jul	177	48.2
Aug	170	33.2
Sep	143	113
Oct	122	86.9
Nov	107	77.5
Dec	106	75.7

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

909 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 909 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 909 would be longer than this page and unusable with a screen reader.
Period	Green Mountain Reservoir (m)
1945.75	2,421
1946.0833333333333	2,408
1946.4166666666667	2,416
1946.6666666666667	2,423
1947	2,414
1947.3333333333333	2,410
1947.6666666666667	2,423
1948	2,416
1948.25	2,403
1948.5833333333333	2,422
1948.9166666666667	2,416
1949.25	2,403
1949.5833333333333	2,423
1949.8333333333333	2,423
1950.1666666666667	2,412
1950.5	2,423
1950.8333333333333	2,418
1951.1666666666667	2,407
1951.4166666666667	2,407
1951.75	2,421
1952.0833333333333	2,415
1952.4166666666667	2,418
1952.75	2,421
1953	2,416
1953.3333333333333	2,404
1953.6666666666667	2,410
1954	2,413
1954.3333333333333	2,403
1954.5833333333333	2,411
1954.9166666666667	2,402
1955.25	2,400
1955.5833333333333	2,422
1955.9166666666667	2,413
1956.1666666666667	2,403
1956.5	2,423
1956.8333333333333	2,409
1957.1666666666667	2,405
1957.5	2,423
1957.75	2,422
1958.0833333333333	2,415
1958.4166666666667	2,421
1958.75	2,418
1959.0833333333333	2,407
1959.3333333333333	2,401
1959.6666666666667	2,421
1960	2,415
1960.3333333333333	2,405
1960.6666666666667	2,421
1960.9166666666667	2,415
1961.25	2,406
1961.5833333333333	2,422
1961.9166666666667	2,421
1962.25	2,398
1962.5	2,422
1962.8333333333333	2,400
1963.1666666666667	2,379
1963.5	2,414
1963.8333333333333	2,408
1964.0833333333333	2,407
1964.4166666666667	2,414
1964.75	2,416
1965.0833333333333	2,410
1965.4166666666667	2,414
1965.6666666666667	2,422
1966	2,416
1966.3333333333333	2,412
1966.6666666666667	2,418
1967	2,410
1967.25	2,402
1967.5833333333333	2,422
1967.9166666666667	2,415
1968.25	2,404
1968.5833333333333	2,423
1968.8333333333333	2,418
1969.1666666666667	2,409
1969.5	2,423
1969.8333333333333	2,418
1970.1666666666667	2,407
1970.4166666666667	2,414
1970.75	2,421
1971.0833333333333	2,412
1971.4166666666667	2,415
1971.75	2,420
1972	2,415
1972.3333333333333	2,405
1972.6666666666667	2,421
1973	2,414
1973.3333333333333	2,408
1973.5833333333333	2,423
1973.9166666666667	2,415
1974.25	2,402
1974.5833333333333	2,423
1974.9166666666667	2,414
1975.1666666666667	2,408
1975.5	2,421
1975.8333333333333	2,417
1976.1666666666667	2,409
1976.5	2,420
1976.75	2,418
1977.0833333333333	2,410
1977.4166666666667	2,417
1977.75	2,407
1978.0833333333333	2,403
1978.3333333333333	2,406
1978.6666666666667	2,420
1979	2,411
1979.3333333333333	2,408
1979.6666666666667	2,422
1979.9166666666667	2,415
1980.25	2,404
1980.5833333333333	2,422
1985.9166666666667	2,417
1986.25	2,400
1986.5	2,420
1986.8333333333333	2,420
1987.1666666666667	2,408
1987.5	2,422
1987.8333333333333	2,415
1988.0833333333333	2,410
1988.4166666666667	2,417
1988.75	2,412
1989.0833333333333	2,407
1989.3333333333333	2,410
1989.6666666666667	2,417
1990	2,407
1990.3333333333333	2,406
1990.6666666666667	2,413
1990.9166666666667	2,407
1991.25	2,406
1991.5833333333333	2,423
1991.9166666666667	2,415
1992.25	2,408
1992.5	2,422
1992.8333333333333	2,413
1993.1666666666667	2,409
1993.5	2,422
1993.8333333333333	2,419
1994.0833333333333	2,412
1994.4166666666667	2,416
1994.75	2,411
1995.0833333333333	2,407
1995.4166666666667	2,419
1995.6666666666667	2,423
1996.0833333333333	2,415
1996.5	2,423
1996.8333333333333	2,418
1997.1666666666667	2,409
1997.4166666666667	2,421
1997.75	2,420
1998.0833333333333	2,406
1998.4166666666667	2,405
1998.75	2,403
1999	2,409
1999.3333333333333	2,409
1999.6666666666667	2,421
2000	2,412
2000.3333333333333	2,411
2000.5833333333333	2,420
2000.9166666666667	2,404
2001.25	2,402
2001.5833333333333	2,421
2001.9166666666667	2,410
2002.1666666666667	2,408
2002.5	2,408
2002.8333333333333	2,400
2003.1666666666667	2,397
2003.5	2,422
2003.75	2,413
2004.0833333333333	2,407
2004.4166666666667	2,415
2004.75	2,413
2005.0833333333333	2,409
2005.3333333333333	2,412
2005.6666666666667	2,419
2006	2,412
2006.3333333333333	2,411
2006.6666666666667	2,418
2006.9166666666667	2,415
2007.25	2,408
2007.5833333333333	2,422
2007.9166666666667	2,412
2008.25	2,405
2008.5	2,423
2008.8333333333333	2,409
2009.1666666666667	2,406
2009.5	2,423
2009.8333333333333	2,411
2010.0833333333333	2,410
2010.4166666666667	2,422
2010.75	2,413
2011.0833333333333	2,409
2011.4166666666667	2,415
2011.6666666666667	2,422
2012	2,412
2012.3333333333333	2,413
2012.6666666666667	2,410
2013	2,406
2013.25	2,406
2013.5833333333333	2,419
2013.9166666666667	2,413
2014.25	2,406
2014.5833333333333	2,422
2014.8333333333333	2,412
2015.1666666666667	2,407
2015.5	2,423
2015.8333333333333	2,409
2016.1666666666667	2,406
2016.4166666666667	2,419
2016.75	2,413
2017.0833333333333	2,407
2017.4166666666667	2,418
2017.75	2,413
2018	2,408
2018.3333333333333	2,409
2018.6666666666667	2,410
2019	2,404
2019.3333333333333	2,408
2019.5833333333333	2,423
2019.9166666666667	2,409
2020.25	2,407
2020.5833333333333	2,420
2020.9166666666667	2,409
2021.1666666666667	2,407
2021.5	2,415
2021.8333333333333	2,407
2022.1666666666667	2,404
2022.5	2,419
2022.75	2,414
2023.0833333333333	2,410
2023.4166666666667	2,421
2023.75	2,414
2024.0833333333333	2,409
2024.3333333333333	2,412
2024.6666666666667	2,419
2025	2,408
2025.3333333333333	2,406
2025.6666666666667	2,413
2025.9166666666667	2,405
2026.25	2,403
2026.5833333333333	2,393

Rate and persistence

-8.07 Mm³

Mean drawdown per month over 12 months

14 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

Green Mountain 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	1945–2026
Completeness	91.9% 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	92% of the months between 1945 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-353.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), Green Mountain Reservoir reservoir records, accessed 2026-09-02, hydrionis.com/reservoirs/reservoir-usbr-353/

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 Green Mountain 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

904 monthly observations 1945–2026 Mm³

Spatially aggregatedHigh confidenceAugust 2026

Source: USBR — Reservoir operations and Colorado River data, RISE catalogue item 21. 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.