Green Mountain Reservoir — storage

Storage now, against this reservoir's own record

23.66 Mm³

Stored, September 2026

0th

Percentile among 76 Seps on record

-83.5%

Against the Sep 1991–2020 average

-79.0%

Change over the 12 months to 2026-09

Storage in September 2026 was lower than almost every other Sep on record, ranked against 76 other Seps held for this reservoir. It is the lowest in the held record for this month.

Storage, 1945–2026

Monthly mean storage

905 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 905 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 905 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
November 1950	150
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
September 1953	160
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
July 1956	188
October 1956	107
February 1957	78.3
June 1957	87.4
October 1957	181
February 1958	129
May 1958	71.3
September 1958	167
January 1959	106
May 1959	59.1
August 1959	186
December 1959	149
April 1960	76.4
August 1960	187
December 1960	129
March 1961	85.4
July 1961	177
November 1961	185
March 1962	80.2
June 1962	95.9
October 1962	123
May 1963	33.6
September 1963	128
January 1964	85.1
April 1964	75.9
August 1964	165
December 1964	116
April 1965	69.5
August 1965	185
November 1965	168
March 1966	96.3
July 1966	188
November 1966	115
February 1967	83.3
June 1967	105
October 1967	149
February 1968	105
June 1968	130
September 1968	182
January 1969	121
May 1969	103
September 1969	170
December 1969	136
April 1970	59.9
August 1970	183
December 1970	145
April 1971	70.9
July 1971	186
November 1971	150
March 1972	96.0
July 1972	184
November 1972	148
February 1973	110
June 1973	127
October 1973	159
February 1974	104
May 1974	73.1
September 1974	158
January 1975	108
May 1975	66.3
September 1975	174
December 1975	130
April 1976	77.5
August 1976	179
December 1976	122
March 1977	94.9
July 1977	142
November 1977	79.9
March 1978	59.0
July 1978	184
October 1978	145
February 1979	92.4
June 1979	142
October 1979	156
February 1980	103
May 1980	57.5
September 1980	161
January 1986	119
May 1986	57.4
August 1986	187
December 1986	164
April 1987	73.8
August 1987	181
December 1987	115
March 1988	85.1
July 1988	183
November 1988	104
March 1989	83.5
June 1989	153
October 1989	102
February 1990	81.0
June 1990	134
October 1990	92.0
January 1991	79.8
May 1991	83.9
September 1991	180
January 1992	118
April 1992	85.8
August 1992	171
December 1992	107
April 1993	86.0
August 1993	186
November 1993	158
March 1994	91.8
July 1994	169
November 1994	98.2
March 1995	81.0
June 1995	154
October 1995	184
March 1996	107
August 1996	184
November 1996	152
March 1997	94.6
July 1997	184
November 1997	172
March 1998	98.4
June 1998	72.5
December 1998	89.0
April 1999	84.2
August 1999	184
November 1999	130
March 2000	91.9
July 2000	187
November 2000	73.2
March 2001	62.0
June 2001	126
October 2001	108
February 2002	91.7
June 2002	101
October 2002	62.0
January 2003	50.2
May 2003	70.5
September 2003	159
January 2004	86.7
April 2004	89.2
August 2004	140
December 2004	98.7
April 2005	89.1
August 2005	185
November 2005	127
March 2006	86.9
July 2006	188
November 2006	132
February 2007	102
June 2007	175
October 2007	122
February 2008	99.1
June 2008	147
September 2008	161
January 2009	82.7
May 2009	113
September 2009	149
January 2010	99.0
April 2010	99.1
August 2010	184
December 2010	97.1
April 2011	76.3
July 2011	186
November 2011	125
March 2012	98.9
July 2012	124
November 2012	84.2
February 2013	75.5
June 2013	145
October 2013	132
February 2014	95.5
May 2014	99.0
September 2014	159
January 2015	94.4
May 2015	112
September 2015	154
December 2015	88.9
April 2016	77.2
August 2016	182
December 2016	91.7
April 2017	81.6
July 2017	188
November 2017	95.8
March 2018	83.1
July 2018	179
October 2018	83.1
February 2019	66.3
June 2019	131
October 2019	122
February 2020	87.3
May 2020	104
September 2020	130
January 2021	90.3
May 2021	75.6
August 2021	118
December 2021	81.9
April 2022	63.4
August 2022	153
December 2022	106
March 2023	93.5
July 2023	186
November 2023	103
March 2024	92.1
July 2024	187
October 2024	111
February 2025	82.0
June 2025	124
October 2025	86.9
January 2026	73.5
May 2026	61.1
September 2026	23.7

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	33.1
Sep	143	23.7
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

910 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 910 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 910 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
1956.25	2,400
1956.5	2,423
1956.8333333333333	2,409
1957.1666666666667	2,405
1957.5	2,423
1957.8333333333333	2,421
1958.0833333333333	2,415
1958.4166666666667	2,421
1958.75	2,418
1959.0833333333333	2,407
1959.4166666666667	2,412
1959.6666666666667	2,421
1960	2,415
1960.3333333333333	2,405
1960.6666666666667	2,421
1961	2,412
1961.25	2,406
1961.5833333333333	2,422
1961.9166666666667	2,421
1962.25	2,398
1962.5833333333333	2,423
1962.8333333333333	2,400
1963.1666666666667	2,379
1963.5	2,414
1963.8333333333333	2,408
1964.1666666666667	2,406
1964.4166666666667	2,414
1964.75	2,416
1965.0833333333333	2,410
1965.4166666666667	2,414
1965.75	2,422
1966	2,416
1966.3333333333333	2,412
1966.6666666666667	2,418
1967	2,410
1967.3333333333333	2,403
1967.5833333333333	2,422
1967.9166666666667	2,415
1968.25	2,404
1968.5833333333333	2,423
1968.9166666666667	2,416
1969.1666666666667	2,409
1969.5	2,423
1969.8333333333333	2,418
1970.1666666666667	2,407
1970.5	2,421
1970.75	2,421
1971.0833333333333	2,412
1971.4166666666667	2,415
1971.75	2,420
1972.0833333333333	2,412
1972.3333333333333	2,405
1972.6666666666667	2,421
1973	2,414
1973.3333333333333	2,408
1973.6666666666667	2,422
1973.9166666666667	2,415
1974.25	2,402
1974.5833333333333	2,423
1974.9166666666667	2,414
1975.25	2,405
1975.5833333333333	2,423
1975.8333333333333	2,417
1976.1666666666667	2,409
1976.5	2,420
1976.8333333333333	2,416
1977.1666666666667	2,409
1977.4166666666667	2,417
1977.75	2,407
1978.0833333333333	2,403
1978.4166666666667	2,414
1978.75	2,417
1979	2,411
1979.3333333333333	2,408
1979.6666666666667	2,422
1980	2,413
1980.3333333333333	2,401
1980.5833333333333	2,422
1985.9166666666667	2,417
1986.25	2,400
1986.5833333333333	2,423
1986.9166666666667	2,417
1987.1666666666667	2,408
1987.5	2,422
1987.8333333333333	2,415
1988.1666666666667	2,407
1988.5	2,422
1988.75	2,412
1989.0833333333333	2,407
1989.4166666666667	2,418
1989.75	2,411
1990.0833333333333	2,407
1990.3333333333333	2,406
1990.6666666666667	2,413
1991	2,406
1991.3333333333333	2,407
1991.6666666666667	2,422
1991.9166666666667	2,415
1992.25	2,408
1992.5833333333333	2,421
1992.9166666666667	2,412
1993.25	2,408
1993.5	2,422
1993.8333333333333	2,419
1994.1666666666667	2,409
1994.5	2,421
1994.8333333333333	2,410
1995.0833333333333	2,407
1995.4166666666667	2,419
1995.75	2,422
1996.1666666666667	2,412
1996.5833333333333	2,422
1996.8333333333333	2,418
1997.1666666666667	2,409
1997.5	2,423
1997.8333333333333	2,421
1998.1666666666667	2,410
1998.5	2,398
1998.75	2,403
1999.0833333333333	2,407
1999.4166666666667	2,417
1999.75	2,418
2000.0833333333333	2,411
2000.3333333333333	2,411
2000.6666666666667	2,415
2001	2,404
2001.3333333333333	2,406
2001.6666666666667	2,416
2001.9166666666667	2,410
2002.25	2,408
2002.5833333333333	2,402
2002.9166666666667	2,399
2003.25	2,398
2003.5	2,422
2003.8333333333333	2,409
2004.1666666666667	2,407
2004.5	2,418
2004.8333333333333	2,411
2005.0833333333333	2,409
2005.4166666666667	2,419
2005.75	2,416
2006.0833333333333	2,410
2006.4166666666667	2,421
2006.6666666666667	2,418
2007	2,413
2007.3333333333333	2,414
2007.6666666666667	2,418
2008	2,412
2008.25	2,405
2008.5833333333333	2,423
2008.9166666666667	2,408
2009.25	2,407
2009.5833333333333	2,422
2009.8333333333333	2,411
2010.1666666666667	2,410
2010.5	2,423
2010.8333333333333	2,410
2011.1666666666667	2,408
2011.4166666666667	2,415
2011.75	2,418
2012.0833333333333	2,411
2012.4166666666667	2,416
2012.75	2,408
2013	2,406
2013.3333333333333	2,410
2013.6666666666667	2,415
2014	2,412
2014.3333333333333	2,410
2014.5833333333333	2,422
2014.9166666666667	2,411
2015.25	2,408
2015.5833333333333	2,423
2015.9166666666667	2,408
2016.1666666666667	2,406
2016.5	2,423
2016.8333333333333	2,409
2017.1666666666667	2,406
2017.5	2,423
2017.8333333333333	2,410
2018.0833333333333	2,408
2018.4166666666667	2,419
2018.75	2,407
2019.0833333333333	2,403
2019.4166666666667	2,415
2019.6666666666667	2,420
2020	2,408
2020.3333333333333	2,411
2020.6666666666667	2,415
2021	2,409
2021.25	2,405
2021.5833333333333	2,413
2021.9166666666667	2,407
2022.25	2,402
2022.5833333333333	2,419
2022.8333333333333	2,412
2023.1666666666667	2,409
2023.5	2,423
2023.8333333333333	2,411
2024.1666666666667	2,409
2024.4166666666667	2,420
2024.75	2,412
2025.0833333333333	2,407
2025.4166666666667	2,414
2025.75	2,408
2026	2,405
2026.3333333333333	2,402
2026.6666666666667	2,388

Rate and persistence

-7.44 Mm³

Mean drawdown per month over 12 months

15 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	92.0% 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	92% of the months between 1945 and 2026 are held. Monthly means of the publisher's own daily record.
Last read from the publisher	2026-10-01
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-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 2026-10-05, 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

905 monthly observations 1945–2026 Mm³

Spatially aggregatedHigh confidenceSeptember 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 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.