Lake Walcott — storage

Storage now, against this reservoir's own record

101.22 Mm³

Stored, August 2026

10th

Percentile among 109 Augs on record

-11.6%

Against the Aug 1991–2020 average

+8.5%

Change over the 12 months to 2026-08

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

Storage, 1917–2026

Monthly mean storage

1,304 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 1,304 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 1,304 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
April 1927	118
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
May 1932	116
November 1932	69.7
April 1933	87.3
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
May 1937	118
November 1937	78.2
April 1938	110
October 1938	94.7
March 1939	91.3
September 1939	113
February 1940	79.5
August 1940	88.1
January 1941	2.55
June 1941	110
December 1941	0
May 1942	123
November 1942	73.9
April 1943	60.0
October 1943	47.8
March 1944	109
September 1944	113
February 1945	81.8
August 1945	118
January 1946	89.8
June 1946	119
December 1946	83.5
May 1947	118
November 1947	117
April 1948	118
October 1948	112
March 1949	96.5
September 1949	119
February 1950	85.0
August 1950	118
January 1951	92.1
June 1951	118
December 1951	91.4
May 1952	117
November 1952	87.6
April 1953	111
October 1953	117
March 1954	95.1
September 1954	116
February 1955	84.7
August 1955	118
January 1956	85.0
June 1956	118
December 1956	85.7
May 1957	119
November 1957	84.9
April 1958	118
October 1958	12.6
March 1959	98.1
September 1959	87.7
February 1960	83.3
July 1960	117
January 1961	82.8
June 1961	114
December 1961	42.3
May 1962	127
November 1962	116
April 1963	125
October 1963	112
March 1964	56.4
September 1964	114
February 1965	65.9
July 1965	119
January 1966	52.1
June 1966	115
December 1966	47.2
May 1967	117
November 1967	75.2
April 1968	111
October 1968	104
March 1969	67.1
September 1969	118
February 1970	39.7
July 1970	119
January 1971	63.2
June 1971	119
December 1971	67.9
May 1972	117
November 1972	79.4
April 1973	119
October 1973	68.0
March 1974	114
August 1974	119
February 1975	66.9
July 1975	119
January 1976	73.0
June 1976	118
December 1976	56.0
May 1977	115
November 1977	104
April 1978	119
October 1978	106
March 1979	71.6
August 1979	121
February 1980	70.7
July 1980	122
January 1981	46.4
June 1981	115
December 1981	48.3
May 1982	116
November 1982	89.3
April 1983	112
October 1983	115
March 1984	55.4
August 1984	116
February 1985	69.4
July 1985	117
January 1986	62.1
June 1986	115
December 1986	78.4
May 1987	117
November 1987	47.5
April 1988	108
September 1988	81.3
March 1989	43.4
August 1989	101
February 1990	34.7
July 1990	117
January 1991	40.2
June 1991	117
December 1991	43.4
May 1992	116
November 1992	49.0
April 1993	78.6
September 1993	118
March 1994	73.0
August 1994	114
February 1995	44.8
July 1995	118
March 1996	87.7
August 1996	118
February 1997	79.4
July 1997	118
January 1998	47.6
June 1998	116
November 1998	44.5
May 1999	114
October 1999	91.9
April 2000	117
September 2000	111
March 2001	53.5
August 2001	112
February 2002	44.0
July 2002	117
January 2003	47.5
June 2003	116
November 2003	30.2
May 2004	117
October 2004	27.8
April 2005	68.1
September 2005	111
March 2006	56.8
August 2006	117
February 2007	46.2
July 2007	116
December 2007	48.0
June 2008	115
November 2008	49.8
May 2009	114
October 2009	65.4
April 2010	112
September 2010	116
March 2011	63.5
August 2011	116
February 2012	46.4
July 2012	111
December 2012	34.4
June 2013	108
November 2013	28.2
May 2014	115
October 2014	56.6
April 2015	115
September 2015	113
March 2016	107
August 2016	113
February 2017	112
July 2017	114
December 2017	111
June 2018	115
November 2018	107
May 2019	114
October 2019	111
April 2020	113
September 2020	91.0
March 2021	96.0
August 2021	114
January 2022	111
July 2022	112
December 2022	78.5
June 2023	112
November 2023	84.9
May 2024	115
October 2024	99.4
April 2025	111
September 2025	92.4
March 2026	60.9
August 2026	101

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	92.4
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

911 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 911 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 911 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
1954.8333333333333	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
1959.9166666666667	1,293
1960.25	1,294
1960.5833333333333	1,294
1960.9166666666667	1,293
1961.25	1,294
1961.5	1,294
1961.8333333333333	1,292
1962.1666666666667	1,293
1962.5	1,294
1962.8333333333333	1,294
1963.0833333333333	1,293
1963.4166666666667	1,294
1963.75	1,294
1964.0833333333333	1,292
1964.4166666666667	1,294
1964.75	1,294
1965	1,292
1965.3333333333333	1,294
1965.6666666666667	1,294
1966	1,292
1966.3333333333333	1,294
1966.5833333333333	1,294
1966.9166666666667	1,292
1967.25	1,294
1967.5833333333333	1,294
1967.9166666666667	1,292
1968.1666666666667	1,293
1968.5	1,294
1968.8333333333333	1,292
1969.1666666666667	1,293
1969.5	1,294
1969.75	1,294
1970.0833333333333	1,292
1970.4166666666667	1,294
1970.75	1,294
1971.0833333333333	1,293
1971.3333333333333	1,294
1971.6666666666667	1,294
1972	1,293
1972.3333333333333	1,294
1972.6666666666667	1,294
1973	1,293
1973.25	1,294
1973.5833333333333	1,294
1973.9166666666667	1,292
1974.25	1,294
1974.5833333333333	1,294
1974.8333333333333	1,293
1975.1666666666667	1,293
1975.5	1,294
1975.8333333333333	1,293
1976.1666666666667	1,294
1976.4166666666667	1,294
1976.75	1,294
1977.0833333333333	1,293
1977.4166666666667	1,294
1977.75	1,293
1978	1,293
1978.3333333333333	1,294
1978.6666666666667	1,294
1979	1,293
1979.3333333333333	1,294
1979.5833333333333	1,294
1979.9166666666667	1,292
1980.25	1,294
1980.5833333333333	1,294
1980.9166666666667	1,292
1981.25	1,294
1981.5	1,294
1981.8333333333333	1,293
1982.1666666666667	1,293
1982.5	1,294
1982.8333333333333	1,293
1983.0833333333333	1,293
1983.4166666666667	1,294
1983.75	1,294
1984.0833333333333	1,293
1984.4166666666667	1,294
1984.6666666666667	1,294
1985	1,293
1985.3333333333333	1,294
1985.6666666666667	1,294
1986	1,293
1986.25	1,294
1986.5833333333333	1,294
1986.9166666666667	1,293
1987.25	1,294
1987.5833333333333	1,294
1987.8333333333333	1,292
1988.1666666666667	1,292
1988.5	1,294
1988.8333333333333	1,292
1989.1666666666667	1,292
1989.5	1,294
1989.75	1,293
1990.0833333333333	1,292
1990.4166666666667	1,294
1990.75	1,292
1991.0833333333333	1,292
1991.3333333333333	1,294
1991.6666666666667	1,294
1992	1,292
1992.3333333333333	1,294
1992.6666666666667	1,292
1992.9166666666667	1,292
1993.25	1,293
1993.5833333333333	1,294
1993.9166666666667	1,292
1994.25	1,294
1994.5	1,294
1994.8333333333333	1,292
1995.1666666666667	1,293
1995.5	1,294
1995.8333333333333	1,291
1996.0833333333333	1,292
1996.4166666666667	1,294
1996.75	1,294
1997.0833333333333	1,293
1997.4166666666667	1,294
1997.75	1,293
1998	1,292
1998.3333333333333	1,294
1998.6666666666667	1,294
1999	1,292
1999.3333333333333	1,294
1999.5833333333333	1,294
1999.9166666666667	1,292
2000.25	1,294
2000.5833333333333	1,294
2000.9166666666667	1,292
2001.1666666666667	1,292
2001.5	1,294
2001.8333333333333	1,292
2002.1666666666667	1,292
2002.5	1,294
2002.75	1,292
2003.0833333333333	1,292
2003.4166666666667	1,294
2003.75	1,292
2004.0833333333333	1,292
2004.3333333333333	1,294
2004.6666666666667	1,293
2005	1,292
2005.3333333333333	1,294
2005.6666666666667	1,294
2006	1,292
2006.25	1,294
2006.5833333333333	1,294
2006.9166666666667	1,292
2007.25	1,294
2007.5833333333333	1,294
2007.8333333333333	1,292
2008.1666666666667	1,292
2008.5	1,294
2008.8333333333333	1,292
2009.1666666666667	1,292
2009.4166666666667	1,294
2009.75	1,293
2010.0833333333333	1,292
2010.4166666666667	1,294
2010.75	1,293
2011	1,292
2011.3333333333333	1,294
2011.6666666666667	1,294
2012	1,292
2012.3333333333333	1,294
2012.5833333333333	1,294
2012.9166666666667	1,292
2013.25	1,294
2013.5833333333333	1,294
2013.9166666666667	1,292
2014.25	1,294
2014.5	1,294
2014.8333333333333	1,292
2015.1666666666667	1,293
2015.5	1,294
2015.8333333333333	1,294
2016.0833333333333	1,294
2016.4166666666667	1,294
2016.75	1,294
2017.0833333333333	1,294
2017.4166666666667	1,294
2017.6666666666667	1,294
2018	1,294
2018.3333333333333	1,294
2018.6666666666667	1,294
2019	1,294
2019.25	1,294
2019.5833333333333	1,294
2019.9166666666667	1,294
2020.25	1,294
2020.5833333333333	1,294
2020.8333333333333	1,293
2021.1666666666667	1,293
2021.5	1,294
2021.8333333333333	1,293
2022.1666666666667	1,294
2022.5	1,294
2022.75	1,293
2023.0833333333333	1,293
2023.4166666666667	1,294
2023.75	1,293
2024.0833333333333	1,293
2024.3333333333333	1,294
2024.6666666666667	1,294
2025	1,293
2025.3333333333333	1,294
2025.6666666666667	1,293
2025.9166666666667	1,292
2026.25	1,294
2026.5833333333333	1,294

Rate and persistence

+0.66 Mm³

Mean change per month over 12 months

10 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	98.8% 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	2026-09-01
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-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 2026-09-02, 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

1,304 monthly observations 1917–2026 Mm³

Spatially aggregatedHigh confidenceAugust 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 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.