Nelson Reservoir — storage

Storage now, against this reservoir's own record

70.93 Mm³

Stored, September 2026

49th

Percentile among 44 Seps on record

+1.0%

Against the Sep 1991–2020 average

-8.7%

Change over the 12 months to 2026-09

Storage in September 2026 was around the middle of the Sep record, ranked against 44 other Seps held for this reservoir.

Storage, 1980–2026

Monthly mean storage

521 monthly values, 1980 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 521 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 521 would be longer than this page and unusable with a screen reader.
Period	Nelson Reservoir (Mm³)
October 1980	51.4
December 1980	51.2
February 1981	48.4
May 1981	61.5
July 1981	52.7
September 1981	42.8
November 1981	62.1
January 1982	58.5
March 1982	59.2
June 1982	91.5
August 1982	78.9
October 1982	85.1
December 1982	84.1
February 1983	78.8
April 1983	78.4
July 1983	58.6
September 1983	47.6
November 1983	50.1
January 1984	47.4
March 1984	46.4
June 1984	32.2
August 1984	25.5
October 1984	38.5
December 1984	39.6
February 1985	37.6
April 1985	45.9
July 1985	31.2
September 1985	34.4
November 1985	63.5
January 1986	59.7
March 1986	73.4
May 1986	97.0
August 1986	78.9
October 1986	88.1
December 1986	82.9
February 1987	77.6
April 1987	81.8
July 1987	75.6
September 1987	82.2
November 1987	81.9
January 1988	79.1
March 1988	73.8
May 1988	64.7
August 1988	31.1
October 1988	29.4
December 1988	27.8
February 1989	26.3
April 1989	41.4
June 1989	64.1
September 1989	64.5
November 1989	74.8
January 1990	70.1
March 1990	68.0
May 1990	90.1
July 1990	78.6
October 1990	75.5
December 1990	71.2
February 1991	67.8
April 1991	67.0
June 1991	67.1
September 1991	75.0
June 1994	91.8
August 1994	66.0
October 1994	54.4
December 1994	50.5
February 1995	48.2
May 1995	69.4
July 1995	70.8
September 1995	71.8
November 1995	61.0
January 1996	57.8
March 1996	59.7
June 1996	89.5
August 1996	73.2
October 1996	70.4
December 1996	65.6
February 1997	62.0
May 1997	85.4
July 1997	79.8
September 1997	75.0
November 1997	92.2
January 1998	85.7
March 1998	80.6
June 1998	58.4
August 1998	59.7
October 1998	69.9
December 1998	70.3
February 1999	66.8
April 1999	76.2
July 1999	87.5
September 1999	73.8
November 1999	77.5
January 2000	71.3
March 2000	68.6
June 2000	57.8
August 2000	39.1
October 2000	34.0
December 2000	39.4
February 2001	37.8
April 2001	43.4
July 2001	47.9
September 2001	42.0
November 2001	40.3
January 2002	38.2
March 2002	36.6
May 2002	46.6
August 2002	50.7
October 2002	78.4
December 2002	78.4
February 2003	73.4
April 2003	86.7
July 2003	76.8
September 2003	58.5
November 2003	59.5
January 2004	55.3
March 2004	54.1
May 2004	70.5
August 2004	67.4
October 2004	74.5
December 2004	75.1
February 2005	69.6
April 2005	73.1
June 2005	67.9
September 2005	68.3
November 2005	71.4
January 2006	66.4
March 2006	64.1
May 2006	88.3
July 2006	74.8
October 2006	61.7
December 2006	57.8
February 2007	55.2
April 2007	78.3
June 2007	96.3
September 2007	61.6
November 2007	66.7
January 2008	61.8
March 2008	59.1
May 2008	58.9
July 2008	70.2
October 2008	79.1
December 2008	74.5
February 2009	70.2
April 2009	85.3
June 2009	85.8
August 2009	79.5
November 2009	84.3
January 2010	78.5
March 2010	76.4
May 2010	95.9
July 2010	93.6
October 2010	93.7
December 2010	87.1
February 2011	83.5
April 2011	91.1
June 2011	91.2
August 2011	87.6
November 2011	92.1
January 2012	86.6
March 2012	82.7
May 2012	93.4
July 2012	88.2
September 2012	77.2
December 2012	78.6
February 2013	74.9
April 2013	93.0
June 2013	93.6
August 2013	94.3
November 2013	91.8
January 2014	87.2
March 2014	83.9
May 2014	95.1
July 2014	92.8
September 2014	88.6
December 2014	81.6
February 2015	78.2
April 2015	93.1
June 2015	87.5
August 2015	70.5
October 2015	81.0
January 2016	74.4
March 2016	73.1
May 2016	92.9
July 2016	72.2
September 2016	32.6
December 2016	35.1
February 2017	34.4
April 2017	66.4
June 2017	78.2
August 2017	52.2
October 2017	62.7
January 2018	57.9
March 2018	55.1
May 2018	87.9
July 2018	92.1
September 2018	86.4
November 2018	88.3
February 2019	80.9
April 2019	95.2
June 2019	86.4
August 2019	91.6
October 2019	92.7
December 2019	87.1
March 2020	88.8
May 2020	95.7
July 2020	89.3
September 2020	71.2
November 2020	66.0
February 2021	60.5
April 2021	75.5
June 2021	58.1
August 2021	48.1
October 2021	58.0
December 2021	54.9
March 2022	51.2
May 2022	57.0
July 2022	62.6
September 2022	52.2
November 2022	55.2
January 2023	52.4
April 2023	58.1
June 2023	90.7
August 2023	78.0
October 2023	82.0
December 2023	76.7
March 2024	78.8
May 2024	93.0
July 2024	85.9
September 2024	78.6
November 2024	71.2
January 2025	67.0
April 2025	89.8
June 2025	65.0
August 2025	75.0
October 2025	73.4
December 2025	69.0
February 2026	66.0
May 2026	66.0
July 2026	70.7
September 2026	70.9

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	67.8	67.3
Feb	66.2	66.0
Mar	66.4	65.9
Apr	76.0	70.6
May	82.9	66.0
Jun	79.2	52.8
Jul	78.1	70.7
Aug	68.4	65.9
Sep	70.3	70.9
Oct	72.6	73.4
Nov	71.8	70.5
Dec	69.6	69.0

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

564 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 564 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 564 would be longer than this page and unusable with a screen reader.
Period	Nelson Reservoir (m)
1979.75	676
1979.9166666666667	676
1980.1666666666667	676
1980.3333333333333	675
1980.5	674
1980.75	674
1980.9166666666667	674
1981.0833333333333	674
1981.3333333333333	675
1981.5	674
1981.75	674
1981.9166666666667	675
1982.0833333333333	675
1982.3333333333333	677
1982.5	677
1982.6666666666667	676
1982.9166666666667	676
1983.0833333333333	676
1983.25	676
1983.5	675
1983.6666666666667	674
1983.8333333333333	674
1984.0833333333333	674
1984.25	674
1984.5	672
1984.6666666666667	671
1984.8333333333333	673
1985.0833333333333	673
1985.25	673
1985.4166666666667	673
1985.6666666666667	672
1985.8333333333333	675
1986	675
1986.25	677
1986.4166666666667	677
1986.5833333333333	676
1986.8333333333333	676
1987	676
1987.25	676
1987.4166666666667	676
1987.5833333333333	676
1987.8333333333333	676
1988	676
1988.1666666666667	676
1988.4166666666667	674
1988.5833333333333	672
1988.75	672
1989	671
1989.1666666666667	672
1989.3333333333333	674
1989.5833333333333	674
1989.75	676
1989.9166666666667	676
1990.1666666666667	675
1990.3333333333333	677
1990.5833333333333	676
1990.75	676
1990.9166666666667	676
1991.1666666666667	675
1991.3333333333333	675
1991.5	676
1991.75	676
1991.9166666666667	676
1992.0833333333333	675
1992.3333333333333	674
1992.5	674
1992.6666666666667	673
1992.9166666666667	673
1993.0833333333333	673
1993.3333333333333	675
1993.5	676
1993.6666666666667	677
1993.9166666666667	676
1994.0833333333333	676
1994.25	677
1994.5	676
1994.6666666666667	675
1994.8333333333333	674
1995.0833333333333	674
1995.25	674
1995.4166666666667	675
1995.6666666666667	676
1995.8333333333333	675
1996.0833333333333	674
1996.25	676
1996.4166666666667	677
1996.6666666666667	675
1996.8333333333333	675
1997	675
1997.25	676
1997.4166666666667	676
1997.5833333333333	675
1997.8333333333333	677
1998	676
1998.1666666666667	676
1998.4166666666667	675
1998.5833333333333	675
1998.75	675
1999	675
1999.1666666666667	675
1999.4166666666667	677
1999.5833333333333	675
1999.75	676
2000	675
2000.1666666666667	675
2000.3333333333333	675
2000.5833333333333	673
2000.75	672
2000.9166666666667	673
2001.1666666666667	672
2001.3333333333333	674
2001.5	674
2001.75	673
2001.9166666666667	673
2002.1666666666667	672
2002.3333333333333	674
2002.5	674
2002.75	676
2002.9166666666667	676
2003.0833333333333	676
2003.3333333333333	677
2003.5	676
2003.6666666666667	675
2003.9166666666667	674
2004.0833333333333	674
2004.25	675
2004.5	676
2004.6666666666667	675
2004.9166666666667	676
2005.0833333333333	675
2005.25	676
2005.5	675
2005.6666666666667	675
2005.8333333333333	676
2006.0833333333333	675
2006.25	676
2006.4166666666667	676
2006.6666666666667	674
2006.8333333333333	675
2007	674
2007.25	676
2007.4166666666667	677
2007.6666666666667	675
2007.8333333333333	675
2008	675
2008.25	675
2008.4166666666667	675
2008.5833333333333	675
2008.8333333333333	676
2009	676
2009.1666666666667	675
2009.4166666666667	676
2009.5833333333333	676
2009.75	677
2010	676
2010.1666666666667	676
2010.3333333333333	677
2010.5833333333333	677
2010.75	677
2011	676
2011.1666666666667	676
2011.3333333333333	677
2011.5833333333333	677
2011.75	677
2011.9166666666667	677
2012.1666666666667	676
2012.3333333333333	677
2012.5	677
2012.75	676
2012.9166666666667	676
2013.0833333333333	676
2013.3333333333333	677
2013.5	677
2013.75	677
2013.9166666666667	677
2014.0833333333333	676
2014.3333333333333	677
2014.5	677
2014.6666666666667	677
2014.9166666666667	676
2015.0833333333333	676
2015.25	677
2015.5	676
2015.6666666666667	676
2015.8333333333333	676
2016.0833333333333	676
2016.25	676
2016.5	676
2016.6666666666667	672
2016.8333333333333	672
2017.0833333333333	672
2017.25	675
2017.4166666666667	676
2017.6666666666667	675
2017.8333333333333	675
2018	675
2018.25	675
2018.4166666666667	677
2018.5833333333333	676
2018.8333333333333	677
2019	676
2019.1666666666667	676
2019.4166666666667	676
2019.5833333333333	677
2019.8333333333333	677
2020	676
2020.1666666666667	677
2020.4166666666667	677
2020.5833333333333	676
2020.75	675
2021	675
2021.1666666666667	675
2021.3333333333333	676
2021.5833333333333	674
2021.75	675
2021.9166666666667	674
2022.1666666666667	674
2022.3333333333333	674
2022.5833333333333	674
2022.75	674
2022.9166666666667	674
2023.1666666666667	674
2023.3333333333333	676
2023.5	677
2023.75	676
2023.9166666666667	676
2024.0833333333333	676
2024.3333333333333	677
2024.5	676
2024.6666666666667	676
2024.9166666666667	675
2025.0833333333333	675
2025.3333333333333	676
2025.5	675
2025.6666666666667	676
2025.9166666666667	675
2026.0833333333333	675
2026.25	675
2026.5	675
2026.6666666666667	675

Rate and persistence

-0.56 Mm³

Mean drawdown per month over 12 months

1 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

Nelson 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	1980–2026
Completeness	92.4% 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 1980 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-424.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), Nelson Reservoir reservoir records, accessed 2026-10-05, hydrionis.com/reservoirs/reservoir-usbr-424/

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 Nelson 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

521 monthly observations 1980–2026 Mm³

Spatially aggregatedHigh confidenceSeptember 2026

Source: USBR — Reservoir operations and Colorado River data, RISE catalogue item 618. 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.