Lake Walcott — storage

Storage now, against this reservoir's own record

80.08 Mm³

Stored, September 2026

18th

Percentile among 109 Seps on record

-21.2%

Against the Sep 1991–2020 average

-13.3%

Change over the 12 months to 2026-09

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

Storage, 1917–2026

Monthly mean storage

1,305 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 1,305 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 1,305 would be longer than this page and unusable with a screen reader.
Period	Lake Walcott (Mm³)
October 1917	96.5
March 1918	105
September 1918	125
February 1919	101
August 1919	105
January 1920	99.5
July 1920	126
December 1920	103
June 1921	125
November 1921	104
May 1922	115
October 1922	74.6
March 1923	110
September 1923	56.6
February 1924	111
August 1924	41.0
January 1925	111
July 1925	130
December 1925	112
June 1926	129
November 1926	52.4
May 1927	115
October 1927	125
March 1928	115
September 1928	114
March 1929	35.5
September 1929	113
February 1930	84.7
August 1930	116
January 1931	77.4
July 1931	117
December 1931	76.1
June 1932	118
November 1932	69.7
May 1933	111
October 1933	60.8
March 1934	92.1
September 1934	24.7
February 1935	77.7
August 1935	92.5
January 1936	78.0
July 1936	118
December 1936	71.7
June 1937	117
November 1937	78.2
May 1938	115
October 1938	94.7
March 1939	91.3
September 1939	113
February 1940	79.5
August 1940	88.1
January 1941	2.55
July 1941	96.0
December 1941	0
June 1942	121
November 1942	73.9
May 1943	84.6
October 1943	47.8
March 1944	109
September 1944	113
February 1945	81.8
August 1945	118
January 1946	89.8
July 1946	119
December 1946	83.5
June 1947	120
November 1947	117
May 1948	118
October 1948	112
March 1949	96.5
September 1949	119
February 1950	85.0
August 1950	118
January 1951	92.1
July 1951	117
December 1951	91.4
June 1952	118
November 1952	87.6
May 1953	116
October 1953	117
March 1954	95.1
September 1954	116
February 1955	84.7
August 1955	118
January 1956	85.0
July 1956	118
December 1956	85.7
June 1957	117
November 1957	84.9
May 1958	117
October 1958	12.6
April 1959	112
September 1959	87.7
February 1960	83.3
August 1960	115
January 1961	82.8
July 1961	114
December 1961	42.3
June 1962	121
November 1962	116
May 1963	119
October 1963	112
April 1964	87.9
September 1964	114
February 1965	65.9
August 1965	119
January 1966	52.1
July 1966	117
December 1966	47.2
June 1967	117
November 1967	75.2
May 1968	115
October 1968	104
April 1969	110
September 1969	118
February 1970	39.7
August 1970	119
January 1971	63.2
July 1971	118
December 1971	67.9
June 1972	118
November 1972	79.4
May 1973	118
October 1973	68.0
April 1974	117
September 1974	120
February 1975	66.9
August 1975	121
January 1976	73.0
July 1976	119
December 1976	56.0
June 1977	116
November 1977	104
May 1978	124
October 1978	106
April 1979	109
September 1979	120
February 1980	70.7
August 1980	121
January 1981	46.4
July 1981	116
December 1981	48.3
June 1982	115
November 1982	89.3
May 1983	118
October 1983	115
April 1984	108
September 1984	116
February 1985	69.4
August 1985	117
January 1986	62.1
July 1986	117
December 1986	78.4
June 1987	117
November 1987	47.5
May 1988	114
October 1988	29.0
April 1989	90.4
September 1989	104
March 1990	38.6
August 1990	114
January 1991	40.2
July 1991	117
December 1991	43.4
June 1992	117
November 1992	49.0
May 1993	116
October 1993	80.2
April 1994	117
September 1994	53.7
March 1995	64.1
August 1995	117
March 1996	87.7
September 1996	114
February 1997	79.4
August 1997	117
January 1998	47.6
July 1998	117
December 1998	41.0
June 1999	116
November 1999	48.6
May 2000	116
October 2000	63.2
March 2001	53.5
September 2001	78.5
February 2002	44.0
August 2002	112
January 2003	47.5
July 2003	116
December 2003	35.1
June 2004	117
November 2004	20.4
May 2005	115
October 2005	59.6
March 2006	56.8
September 2006	117
February 2007	46.2
August 2007	116
January 2008	48.0
July 2008	114
December 2008	50.1
June 2009	116
November 2009	47.4
May 2010	116
October 2010	58.7
March 2011	63.5
September 2011	116
February 2012	46.4
August 2012	110
January 2013	41.1
July 2013	110
December 2013	31.4
June 2014	113
November 2014	50.4
May 2015	115
October 2015	111
April 2016	109
September 2016	115
February 2017	112
August 2017	112
January 2018	113
July 2018	111
December 2018	105
June 2019	111
November 2019	107
May 2020	116
October 2020	43.3
April 2021	107
September 2021	108
February 2022	115
August 2022	111
January 2023	82.1
July 2023	111
December 2023	87.8
June 2024	112
November 2024	96.9
May 2025	112
October 2025	51.6
April 2026	108
September 2026	80.1

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	52.7	48.4
Feb	55.8	49.1
Mar	66.5	60.9
Apr	107	108
May	115	106
Jun	115	103
Jul	115	108
Aug	115	101
Sep	102	80.1
Oct	66.3	51.6
Nov	55.7	39.8
Dec	56.1	46.3

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

912 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 912 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 912 would be longer than this page and unusable with a screen reader.
Period	Lake Walcott (m)
1950.75	1,294
1951.0833333333333	1,293
1951.4166666666667	1,294
1951.6666666666667	1,294
1952	1,293
1952.3333333333333	1,294
1952.6666666666667	1,294
1953	1,293
1953.25	1,294
1953.5833333333333	1,294
1953.9166666666667	1,293
1954.25	1,294
1954.5833333333333	1,294
1954.9166666666667	1,293
1955.1666666666667	1,293
1955.5	1,294
1955.8333333333333	1,294
1956.1666666666667	1,293
1956.5	1,294
1956.75	1,294
1957.0833333333333	1,293
1957.4166666666667	1,294
1957.75	1,294
1958.0833333333333	1,293
1958.3333333333333	1,294
1958.6666666666667	1,293
1959	1,293
1959.3333333333333	1,294
1959.6666666666667	1,293
1960	1,293
1960.25	1,294
1960.5833333333333	1,294
1960.9166666666667	1,293
1961.25	1,294
1961.5833333333333	1,294
1961.8333333333333	1,292
1962.1666666666667	1,293
1962.5	1,294
1962.8333333333333	1,294
1963.1666666666667	1,293
1963.4166666666667	1,294
1963.75	1,294
1964.0833333333333	1,292
1964.4166666666667	1,294
1964.75	1,294
1965.0833333333333	1,293
1965.3333333333333	1,294
1965.6666666666667	1,294
1966	1,292
1966.3333333333333	1,294
1966.6666666666667	1,293
1966.9166666666667	1,292
1967.25	1,294
1967.5833333333333	1,294
1967.9166666666667	1,292
1968.25	1,294
1968.5	1,294
1968.8333333333333	1,292
1969.1666666666667	1,293
1969.5	1,294
1969.8333333333333	1,293
1970.1666666666667	1,292
1970.4166666666667	1,294
1970.75	1,294
1971.0833333333333	1,293
1971.4166666666667	1,294
1971.75	1,294
1972	1,293
1972.3333333333333	1,294
1972.6666666666667	1,294
1973	1,293
1973.3333333333333	1,294
1973.5833333333333	1,294
1973.9166666666667	1,292
1974.25	1,294
1974.5833333333333	1,294
1974.9166666666667	1,293
1975.25	1,294
1975.5	1,294
1975.8333333333333	1,293
1976.1666666666667	1,294
1976.5	1,294
1976.8333333333333	1,293
1977.0833333333333	1,293
1977.4166666666667	1,294
1977.75	1,293
1978.0833333333333	1,293
1978.4166666666667	1,294
1978.6666666666667	1,294
1979	1,293
1979.3333333333333	1,294
1979.6666666666667	1,294
1980	1,293
1980.25	1,294
1980.5833333333333	1,294
1980.9166666666667	1,292
1981.25	1,294
1981.5833333333333	1,294
1981.9166666666667	1,292
1982.1666666666667	1,293
1982.5	1,294
1982.8333333333333	1,293
1983.1666666666667	1,293
1983.5	1,294
1983.75	1,294
1984.0833333333333	1,293
1984.4166666666667	1,294
1984.75	1,294
1985.0833333333333	1,293
1985.3333333333333	1,294
1985.6666666666667	1,294
1986	1,293
1986.3333333333333	1,294
1986.6666666666667	1,294
1987	1,293
1987.25	1,294
1987.5833333333333	1,294
1987.9166666666667	1,292
1988.25	1,294
1988.5833333333333	1,294
1988.8333333333333	1,292
1989.1666666666667	1,292
1989.5	1,294
1989.8333333333333	1,292
1990.1666666666667	1,292
1990.4166666666667	1,294
1990.75	1,292
1991.0833333333333	1,292
1991.4166666666667	1,294
1991.75	1,292
1992.0833333333333	1,292
1992.3333333333333	1,294
1992.6666666666667	1,292
1993	1,292
1993.3333333333333	1,294
1993.6666666666667	1,294
1993.9166666666667	1,292
1994.25	1,294
1994.5833333333333	1,294
1994.9166666666667	1,292
1995.25	1,294
1995.5	1,294
1995.8333333333333	1,291
1996.1666666666667	1,293
1996.5	1,294
1996.8333333333333	1,293
1997.1666666666667	1,293
1997.4166666666667	1,294
1997.75	1,293
1998.0833333333333	1,292
1998.4166666666667	1,294
1998.75	1,293
1999	1,292
1999.3333333333333	1,294
1999.6666666666667	1,294
2000	1,292
2000.3333333333333	1,294
2000.5833333333333	1,294
2000.9166666666667	1,292
2001.25	1,294
2001.5833333333333	1,294
2001.9166666666667	1,292
2002.1666666666667	1,292
2002.5	1,294
2002.8333333333333	1,292
2003.1666666666667	1,292
2003.5	1,294
2003.8333333333333	1,292
2004.0833333333333	1,292
2004.4166666666667	1,294
2004.75	1,292
2005.0833333333333	1,292
2005.4166666666667	1,294
2005.6666666666667	1,294
2006	1,292
2006.3333333333333	1,294
2006.6666666666667	1,294
2007	1,292
2007.25	1,294
2007.5833333333333	1,294
2007.9166666666667	1,292
2008.25	1,293
2008.5833333333333	1,294
2008.9166666666667	1,292
2009.1666666666667	1,292
2009.5	1,294
2009.8333333333333	1,292
2010.1666666666667	1,292
2010.5	1,294
2010.75	1,293
2011.0833333333333	1,292
2011.4166666666667	1,294
2011.75	1,293
2012.0833333333333	1,292
2012.3333333333333	1,294
2012.6666666666667	1,294
2013	1,292
2013.3333333333333	1,294
2013.6666666666667	1,293
2014	1,292
2014.25	1,294
2014.5833333333333	1,294
2014.9166666666667	1,292
2015.25	1,294
2015.5833333333333	1,294
2015.8333333333333	1,294
2016.1666666666667	1,294
2016.5	1,294
2016.8333333333333	1,294
2017.1666666666667	1,294
2017.4166666666667	1,294
2017.75	1,294
2018.0833333333333	1,294
2018.4166666666667	1,294
2018.75	1,294
2019.0833333333333	1,294
2019.3333333333333	1,294
2019.6666666666667	1,294
2020	1,294
2020.3333333333333	1,294
2020.6666666666667	1,293
2020.9166666666667	1,293
2021.25	1,294
2021.5833333333333	1,294
2021.9166666666667	1,294
2022.25	1,294
2022.5	1,294
2022.8333333333333	1,293
2023.1666666666667	1,293
2023.5	1,294
2023.8333333333333	1,293
2024.1666666666667	1,294
2024.4166666666667	1,294
2024.75	1,293
2025.0833333333333	1,293
2025.4166666666667	1,294
2025.75	1,292
2026	1,292
2026.3333333333333	1,294
2026.6666666666667	1,293

Rate and persistence

-1.03 Mm³

Mean drawdown per month over 12 months

11 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

Lake Walcott

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	98.9% 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	99% of the months between 1917 and 2026 are held. Monthly means of the publisher's own daily record.
Last read from the publisher	2026-10-01
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-7191.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), Lake Walcott reservoir records, accessed 2026-10-05, hydrionis.com/reservoirs/reservoir-usbr-7191/

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 Lake Walcott 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

1,305 monthly observations 1917–2026 Mm³

Spatially aggregatedHigh confidenceSeptember 2026

Source: USBR — Reservoir operations and Colorado River data, RISE catalogue item 131804. 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.