Edward Arthur Patterson Lake — storage

Storage now, against this reservoir's own record

7.23 Mm³

Stored, September 2026

49th

Percentile among 75 Seps on record

-4.8%

Against the Sep 1991–2020 average

-6.9%

Change over the 12 months to 2026-09

Storage in September 2026 was around the middle of the Sep record, ranked against 75 other Seps held for this reservoir.

Storage, 1951–2026

Monthly mean storage

900 monthly values, 1951 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 900 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 900 would be longer than this page and unusable with a screen reader.
Period	Edward Arthur Patterson Lake (Mm³)
October 1951	0
February 1952	0
June 1952	7.53
September 1952	5.94
January 1953	5.17
May 1953	7.43
September 1953	8.09
December 1953	6.87
April 1954	9.00
August 1954	5.38
December 1954	4.52
March 1955	5.73
July 1955	8.11
November 1955	5.12
March 1956	5.62
June 1956	6.59
October 1956	5.59
February 1957	5.32
June 1957	8.30
September 1957	7.41
January 1958	6.62
May 1958	7.79
September 1958	6.53
January 1959	5.86
April 1959	8.31
August 1959	5.87
December 1959	6.11
April 1960	8.19
July 1960	7.84
November 1960	5.63
March 1961	6.12
July 1961	5.20
October 1961	4.22
February 1962	3.74
June 1962	8.45
October 1962	6.62
January 1963	6.09
May 1963	8.53
September 1963	6.56
January 1964	5.45
April 1964	5.39
August 1964	7.31
December 1964	6.05
April 1965	8.50
August 1965	7.68
November 1965	6.98
March 1966	8.11
July 1966	8.28
November 1966	6.72
February 1967	6.85
June 1967	8.14
October 1967	5.41
February 1968	5.15
May 1968	5.92
September 1968	5.69
January 1969	5.26
May 1969	7.99
August 1969	7.61
December 1969	6.21
April 1970	8.24
August 1970	7.45
November 1970	6.57
March 1971	8.71
July 1971	7.89
November 1971	8.05
February 1972	8.19
June 1972	8.38
October 1972	7.74
February 1973	8.37
June 1973	8.21
September 1973	5.95
January 1974	6.20
May 1974	7.79
September 1974	5.89
December 1974	5.48
April 1975	7.27
August 1975	7.10
December 1975	6.21
March 1976	8.39
July 1976	7.84
November 1976	5.33
March 1977	5.88
June 1977	8.51
October 1977	8.43
February 1978	8.24
June 1978	8.38
September 1978	7.16
January 1979	7.16
May 1979	8.28
September 1979	6.60
January 1980	5.97
April 1980	7.44
August 1980	4.90
December 1980	4.62
April 1981	4.37
July 1981	4.71
November 1981	7.09
March 1982	10.7
July 1982	12.0
October 1982	12.2
February 1983	13.1
June 1983	12.4
October 1983	8.37
January 1984	7.86
May 1984	12.7
September 1984	10.3
January 1985	9.74
April 1985	10.6
August 1985	8.02
December 1985	7.42
April 1986	12.8
August 1986	12.2
November 1986	12.6
March 1987	12.9
July 1987	11.5
November 1987	11.0
February 1988	10.7
June 1988	10.1
October 1988	6.37
February 1989	5.67
May 1989	12.2
September 1989	9.18
January 1990	7.93
May 1990	7.54
August 1990	7.82
December 1990	5.80
April 1991	5.55
August 1991	4.27
November 1991	3.28
March 1992	3.63
July 1992	3.39
November 1992	2.59
March 1993	4.87
June 1993	6.55
October 1993	7.76
February 1994	7.86
June 1994	12.9
September 1994	10.6
January 1995	11.2
May 1995	13.1
September 1995	13.1
December 1995	10.9
April 1996	10.9
August 1996	7.71
December 1996	7.46
March 1997	11.2
July 1997	10.8
November 1997	8.45
March 1998	9.22
June 1998	10.1
October 1998	9.69
February 1999	10.8
June 1999	9.54
October 1999	7.44
January 2000	7.92
May 2000	8.35
September 2000	5.99
January 2001	6.35
April 2001	10.8
August 2001	10.3
December 2001	9.28
April 2002	10.2
July 2002	9.86
November 2002	7.73
March 2003	9.94
July 2003	8.29
October 2003	7.76
February 2004	8.09
June 2004	9.91
October 2004	7.14
January 2005	6.38
May 2005	8.60
September 2005	3.63
January 2006	2.66
May 2006	9.12
August 2006	7.13
December 2006	6.10
April 2007	7.51
August 2007	7.81
November 2007	5.68
March 2008	5.64
July 2008	4.93
November 2008	3.54
February 2009	2.93
June 2009	10.8
October 2009	1.75
February 2010	1.68
May 2010	10.9
September 2010	9.52
January 2011	9.87
May 2011	11.3
August 2011	10.7
December 2011	9.27
April 2012	10.5
August 2012	8.62
November 2012	7.27
March 2013	8.64
July 2013	10.4
November 2013	9.57
March 2014	10.8
June 2014	10.9
October 2014	7.16
February 2015	10.9
June 2015	10.2
September 2015	5.09
January 2016	5.71
May 2016	7.14
September 2016	5.72
December 2016	6.27
April 2017	10.7
August 2017	7.87
December 2017	7.24
March 2018	8.76
July 2018	10.7
November 2018	9.26
March 2019	10.7
June 2019	10.7
October 2019	11.0
February 2020	10.5
June 2020	8.46
October 2020	6.96
January 2021	6.04
May 2021	6.45
September 2021	5.34
January 2022	5.48
April 2022	6.86
August 2022	9.87
December 2022	9.15
April 2023	10.8
July 2023	9.67
November 2023	9.39
March 2024	9.36
July 2024	8.91
October 2024	7.30
February 2025	7.70
June 2025	9.92
October 2025	7.79
January 2026	7.86
May 2026	8.74
September 2026	7.23

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	7.26	7.86
Feb	7.58	9.73
Mar	8.65	9.77
Apr	9.48	9.22
May	9.48	8.74
Jun	9.35	8.53
Jul	8.79	7.97
Aug	8.05	7.50
Sep	7.60	7.23
Oct	7.19	7.79
Nov	7.09	8.00
Dec	7.11	7.97

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

900 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 900 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 900 would be longer than this page and unusable with a screen reader.
Period	Edward Arthur Patterson Lake (m)
1951.75	0
1952.0833333333333	0
1952.4166666666667	736
1952.6666666666667	736
1953	735
1953.3333333333333	736
1953.6666666666667	736
1953.9166666666667	736
1954.25	737
1954.5833333333333	736
1954.9166666666667	735
1955.1666666666667	736
1955.5	736
1955.8333333333333	735
1956.1666666666667	736
1956.4166666666667	736
1956.75	736
1957.0833333333333	735
1957.4166666666667	736
1957.6666666666667	736
1958	736
1958.3333333333333	736
1958.6666666666667	736
1959	736
1959.25	737
1959.5833333333333	736
1959.9166666666667	736
1960.25	736
1960.5	736
1960.8333333333333	736
1961.1666666666667	736
1961.5	735
1961.75	735
1962.0833333333333	735
1962.4166666666667	737
1962.75	736
1963	736
1963.3333333333333	737
1963.6666666666667	736
1964	736
1964.25	736
1964.5833333333333	736
1964.9166666666667	736
1965.25	737
1965.5833333333333	736
1965.8333333333333	736
1966.1666666666667	736
1966.5	737
1966.8333333333333	736
1967.0833333333333	736
1967.4166666666667	737
1967.75	736
1968.0833333333333	735
1968.3333333333333	736
1968.6666666666667	736
1969	735
1969.3333333333333	736
1969.5833333333333	736
1969.9166666666667	736
1970.25	737
1970.5833333333333	736
1970.8333333333333	736
1971.1666666666667	737
1971.5	736
1971.8333333333333	736
1972.0833333333333	737
1972.4166666666667	737
1972.75	736
1973.0833333333333	737
1973.4166666666667	737
1973.6666666666667	736
1974	736
1974.3333333333333	736
1974.6666666666667	736
1974.9166666666667	736
1975.25	736
1975.5833333333333	736
1975.9166666666667	736
1976.1666666666667	737
1976.5	736
1976.8333333333333	735
1977.1666666666667	736
1977.4166666666667	737
1977.75	737
1978.0833333333333	737
1978.4166666666667	737
1978.6666666666667	736
1979	736
1979.3333333333333	737
1979.6666666666667	736
1980	736
1980.25	736
1980.5833333333333	735
1980.9166666666667	735
1981.25	735
1981.5	735
1981.8333333333333	736
1982.1666666666667	737
1982.5	738
1982.75	738
1983.0833333333333	738
1983.4166666666667	738
1983.75	737
1984	736
1984.3333333333333	738
1984.6666666666667	737
1985	737
1985.25	737
1985.5833333333333	736
1985.9166666666667	736
1986.25	738
1986.5833333333333	738
1986.8333333333333	738
1987.1666666666667	738
1987.5	737
1987.8333333333333	737
1988.0833333333333	737
1988.4166666666667	737
1988.75	736
1989.0833333333333	736
1989.3333333333333	738
1989.6666666666667	737
1990	736
1990.3333333333333	736
1990.5833333333333	736
1990.9166666666667	736
1991.25	736
1991.5833333333333	735
1991.8333333333333	734
1992.1666666666667	735
1992.5	735
1992.8333333333333	734
1993.1666666666667	735
1993.4166666666667	736
1993.75	736
1994.0833333333333	736
1994.4166666666667	738
1994.6666666666667	737
1995	737
1995.3333333333333	738
1995.6666666666667	738
1995.9166666666667	738
1996.25	738
1996.5833333333333	737
1996.9166666666667	737
1997.1666666666667	738
1997.5	738
1997.8333333333333	737
1998.1666666666667	737
1998.4166666666667	737
1998.75	737
1999.0833333333333	738
1999.4166666666667	737
1999.75	737
2000	737
2000.3333333333333	737
2000.6666666666667	736
2001	737
2001.25	738
2001.5833333333333	738
2001.9166666666667	737
2002.25	738
2002.5	737
2002.8333333333333	737
2003.1666666666667	737
2003.5	737
2003.75	737
2004.0833333333333	737
2004.4166666666667	737
2004.75	737
2005	737
2005.3333333333333	737
2005.6666666666667	736
2006	735
2006.3333333333333	737
2006.5833333333333	737
2006.9166666666667	737
2007.25	737
2007.5833333333333	737
2007.8333333333333	736
2008.1666666666667	736
2008.5	736
2008.8333333333333	736
2009.0833333333333	735
2009.4166666666667	738
2009.75	735
2010.0833333333333	734
2010.3333333333333	738
2010.6666666666667	737
2011	737
2011.3333333333333	738
2011.5833333333333	738
2011.9166666666667	737
2012.25	738
2012.5833333333333	737
2012.8333333333333	737
2013.1666666666667	737
2013.5	738
2013.8333333333333	737
2014.1666666666667	738
2014.4166666666667	738
2014.75	737
2015.0833333333333	738
2015.4166666666667	738
2015.6666666666667	736
2016	736
2016.3333333333333	737
2016.6666666666667	736
2016.9166666666667	737
2017.25	738
2017.5833333333333	737
2017.9166666666667	737
2018.1666666666667	737
2018.5	738
2018.8333333333333	737
2019.1666666666667	738
2019.4166666666667	738
2019.75	738
2020.0833333333333	738
2020.4166666666667	737
2020.75	737
2021	736
2021.3333333333333	737
2021.6666666666667	736
2022	736
2022.25	737
2022.5833333333333	737
2022.9166666666667	737
2023.25	738
2023.5	737
2023.8333333333333	737
2024.1666666666667	737
2024.5	737
2024.75	737
2025.0833333333333	737
2025.4166666666667	738
2025.75	737
2026	737
2026.3333333333333	737
2026.6666666666667	737

Rate and persistence

-0.04 Mm³

Mean drawdown per month over 12 months

0 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

Edward Arthur Patterson Lake

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	1951–2026
Completeness	98.7% 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 1951 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-318.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), Edward Arthur Patterson Lake reservoir records, accessed 2026-10-05, hydrionis.com/reservoirs/reservoir-usbr-318/

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 Edward Arthur Patterson Lake 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

900 monthly observations 1951–2026 Mm³

Spatially aggregatedHigh confidenceSeptember 2026

Source: USBR — Reservoir operations and Colorado River data, RISE catalogue item 304. 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.