Part of: Powderhorn Park

Powderhorn Lake is a 12-acre lake in one of the largest parks in the Minneapolis system. Many water quality restoration projects have occurred at Powderhorn since its original purchase in 1890 including: dredging, aeration, groundwater augmentation, shoreline restoration, stormwater best management practices, alum and barley straw treatments. The Minnesota Department of Natural Resources (MnDNR) has stocked the lake with fish as part of the Kid’s Fishing Pond since 1980. Aquatic plants were first noted in surveys in 2006. The presence of aquatic vegetation represents an improvement in water clarity for a lake where the algae growth limited light penetration to the extent that little or no aquatic plants growth had occurred in the past. Powderhorn does not have a swimming beach and was therefore not scored for public health.

Recent Secchi Disk Readings

June 19, 2019

Secchi Depth (feet): 2.8

About Secchi Disk Readings

To measure water transparency with a Secchi disk, the disk is lowered from the shaded side of a boat until it cannot be seen. The depth of the water is recorded at the point where the disk reappears upon raising it from its original depth beyond visibility. It is important to remember that the Secchi measurement is a simple, approximate measurement of water clarity and can be influenced by various factors such as time of day, reader’s eyesight, water color and suspended particles in the water. Therefore, Secchi disk readings should be used as a comparative tool to determine trends between lakes and over time.

  • Increased water clarity may be due to reduced nutrient inputs, seasonal algal cycles, decreased sediment inputs, and zooplankton grazing on algae.
  • Decreased water clarity may be due to large algal blooms, increased soil erosion inputs, wind circulation of sediments and nutrients, stormwater inputs, or a decrease in zooplankton.

Secchi readings for the lakes are done during the regular lake sampling schedule, with some lakes sampled every other year.

Lake Aesthetic and User Recreation Index (LAURI)

The Lake Aesthetic and User Recreation Index (LAURI) was designed to give recreational users a source of information about conditions affecting their use of city lakes.  The goal is to have an accurate, science based and easily understandable recreational indicator for the public.  The  LAURI has five indices:

Public Health Index (E. coli measured at public swimming beaches)

The score for each lake is based on the geometric mean number of Escherichia coli (E. coli) collected by the beach monitoring program (Table 1-1).  Lakes with more than one beach were averaged together.  EPA and Minnesota guidelines state that beaches should not exceed a geometric mean of 126 organisms per 100 mL during a 30-day time period.  Lower numbers of organisms indicate less risk of illnesses for lake users.

Table 1-1.  Scoring for the public health portion of LAURI.

E. coli, (CFU/100mL)* Score
<2 (ND) 10
2-10 9
11-20 8
21-35 7
36-50 6
51-65 5
66-80 4
81-100 3
101-125 2
>126 1

 

 

 

 

 

 

 

 

 

 

 

 

* The value used is the running geometric mean for the year, averaged for all the beaches on a lake.

Water Quality Index (water clarity/Secchi depth)

Water clarity is a good integrator of various parameters affecting the quality of a lake. The average Secchi transparency reading from all the data collected during the growing season is used. The lakes are separated into deep lakes and shallow lakes using criteria developed by the Minnesota Pollution Control Agency (MPCA). A shallow lake is defined as 80% littoral (< 15 feet deep). Bde Maka Ska, Cedar, Harriet, and Wirth are considered deep lakes. Loring, Isles, Hiawatha, Nokomis, and Powderhorn are considered shallow lakes. Greater secchi depth indicates clearer water.  LAURI scoring is shown below in Table 1-2.

Table 1-2.  Scoring for the water quality portion of LAURI.

Secchi Depth (m) Deep Lake Score Shallow Lake Score
0-0.5 1 2
0.6-1 2 4
1.1-1.5 3 6
1.6-2.0 4 8
2.1-2.5 5 10
2.6-3.0 6
3.1-3.5 7
3.6-4.0 8
4.1-4.5 9
>4.6 10

Habitat Quality (aquatic plant and fish diversity)

LAURI assessments of habitat quality are based on detailed Minneapolis Park and Recreation Board (MPRB) surveys and DNR fish sampling. Scoring is based on presence of aquatic plants, density of plants, and amount of coverage (Table 1-3). Lakes with more aquatic plants receive higher habitat quality index scores.  Points are also awarded for more diverse fish populations. Each fish score is based on the most recent available DNR fish survey done for a lake. The score from the aquatic plant and fish surveys are averaged for the LAURI.

Table 1-3. Scoring for the habitat portion of LAURI.

Macrophyte
Species
Score Density Score Coverage
>15 ft.
Score # Fish
Species
Score
0 0 Low 0 0-25 2 <=6 2
1-4 3 Low-Med 3 25-50 4 7-8 4
4-8 6 Medium 6 50-75 7 9-11 6
8-15 8 Med-High 8 75-100 10 12-14 8
>15 10 High 10 >=15 10

Recreational Access (availability and ease of public access)

The lakes are also scored for the quantity and quality of recreational access to the water. The recreational score considers the number of fishing docks, beaches, boat launches, intra lake connections, boat rentals, canoe racks and sail boat buoys, boardwalks, picnic areas and concessions at a lake, Table 1-4. While aquatic plants are a necessary part of a healthy lake ecosystem they can also interfere with recreational uses of the lake.  Lakes also receive points for invasive plant growth management.

Table 1-4. Scoring for the recreational access portion of LAURI.

Recreational Opportunities # RecOps at Site Total # RecOps +
Aquatic Plant Mgmt
Final Score
Fishing Dock 0 1
Beach 1 2
Boat Launches 2 3
Intra Lake Connection 3 4
Boat Rental 4 5
Boat Storage 5 6
Boardwalk 6 7
Picnic Area 7-8 8
Concessions 8-9 9
Aquatic Mgmt = yes (+4) >10 10
Aquatic Mgmt = no (+0)
Total # RecOps at site +
Aquatic Plant Mgmt

Aesthetic Considerations (color and odor of water, garbage and debris)

The lakes are scored for water color, odor, and debris based on an assessment done from shore, dock, or boat, Table 1-5.  Higher numbers indicate better aesthetics.  The scores are averaged over the season.  Aesthetics can be difficult to evaluate because they are qualitative and dependent on individual experience.

Table 1-5.  Scoring for the aesthetic portion of LAURI.

Color Score Odor Score Debris Score
Clear 10 None/Natural 10 None 10
Lt. Brown or
Green
8 Musty-Faint 8 Natural 9
Bright Green 5 Musty-Strong 6 Foam 8
Milky White 4 Sewage/
Fishy/
Garbage-Faint
5 Piles of Milfoil
(>3)
7
Brown/
Reddish/
Purple
2 Sewage/
Fishy/
Garbage-Strong
2 Trash: Fixed
(>3)
4
Gray/Black 0 Anaerobic/Septic 0 Trash: Floating
(>3)
3
Many Dead Fish
(>5)
2
Green Scum 2
Oil Film 1
Sewage Solids 0