Taltson Hydro Project NORTHWEST TERRITORIES POWER Northwest Territories Power Corporation CORPORATION Yellowknife, Northwest Territories

Taltson Hydro Project Meteorology and Hydrology Compilation Data Report

Prepared by: April 2001

Rescan™ Environmental Services Ltd. TM Yellowknife, Northwest Territories EXECUTIVE SUMMARY

TM EXECUTIVE SUMMARY

This report is a compilation of all available and relevant meteorology and hydrology data collected, recorded, and purchased throughout the life of the Taltson Hydro Project. The data are analyzed, summarized, and discussed in the main body of the report, while the raw data are available on the accompanying CD and the 1998 Hydat CD. The purpose of the report is to compile and summarize meteorology/hydrology data and to identify which components of the Project require further investigation in order to meet the goals of the Water Effects Monitoring Program (WEMP).

The primary meteorological parameters used to characterize the Taltson Hydro Project are air temperature and precipitation, both of which have a direct influence on the operation of the Taltson Hydro Project. The nearest meteorological station with a long-term period of record is Fort Smith Airport. Meteorological data from Fort Smith Airport are considered representative for the Taltson area. Temperature and precipitation data from Fort Smith are summarized herein.

All available hydrological data relevant to the WEMP are summarized. The majority of the data can be found in previous reports on the Taltson Hydro Project. The remaining data were obtained through Northwest Territories Power Corporation. The hydrological component of the report summarizes and presents: historic and present flow monitoring by Water Survey of Canada; stage-discharge relationships and weir equations; hydraulic controls of the Taltson River; limnological data from relevant lakes; historic water levels and flows with and without the Twin Gorges Plant and Nonacho Dam in place (1977- 1985); recent operating conditions from Twin Gorges Plant and Nonacho Dam (1987- 2000); flows over the South Valley Spillway; and available aerial photographs.

No documented account of winter freeze-up and summer break-up of ice exists for the Taltson Basin. The general pattern of ice formation and melt is qualitatively discussed in a previous report. River and lake water temperature data from the open-water season are presented.

The WEMP developed for the Taltson Hydro Project identified four key hydrologic issues and the activities required to address these issues. This report highlights components of the hydrologic issues that can be addressed, in part or in full, with existing data from previous environmental monitoring.

The first hydrologic issue is changes in water levels and ice cover characteristics on Nonacho Lake and on lakes upstream of Nonacho Lake (Hjalmar and Gray lakes). Sufficient data exist from pre- and post-dam construction to quantify changes in water levels (and extent of flooding) on Nonacho Lake. However, data from Hjalmar and Gray lakes have not been collected. No ice cover data exist for all lakes of concern (Nonacho, Hjalmar and Gray lakes). Traditional and potential future travel routes have also not been documented.

The second hydrologic issue is ice-related fluctuations on the Taltson River between Elsie Falls and Tsu Lake. This issue refers particularly to freeze-up, mid-winter

Northwest Territories Power Corporation - i - Rescan™ Environmental Services Ltd. EXECUTIVE SUMMARY

overflows and break-up. No ice monitoring data have been collected from the Taltson River Basin. Water temperature data from the open-water season and long-term precipitation data are available.

The third hydrologic issue is atypical water level fluctuations below Nonacho Lake and downstream of Twin Gorges, on Trudel Creek, on the Taltson River between Elsie Falls and Tsu Lake, and on the Taltson River between Tsu Lake and the mouth of the Taltson River. The Taltson Flow Model reconstructed water levels and flow from pre- and post- development at the outlet of all major lakes on the Taltson River. These data will aid in addressing this issue, however, additional data will be required from critical locations along the Taltson River and Trudel Creek. The critical areas also need to be defined through discussions with stakeholders.

The fourth hydrologic issue is sedimentation due to erosion and instability of the channel planform of Trudel Creek and on the Taltson River between Twin Gorges and Tsu Lake and between Tsu Lake and the mouth of the Taltson River. The only available data from Trudel Creek are inflows from the South Valley Spillway and pre- and post-spillway air photos. No pre- or post-spillway geomorphic characteristics have been collected from Trudel Creek. All sedimentation and erosion data required to address the hydrologic issues concerning Trudel Creek are yet to be collected.

In summary, all four hydrologic issues outlined in the WEMP cannot be fully addressed with available data. Details of the monitoring required to properly address these issues are presented in Section 5 of this report, which was based on the WEMP.

Northwest Territories Power Corporation - ii - Rescan™ Environmental Services Ltd. TABLE OF CONTENTS

TM TALTSON HYDRO PROJECT Meteorology and Hydrology Compilation Data Report

TABLE OF CONTENTS

EXECUTIVE SUMMARY ...... i

TABLE OF CONTENTS...... ii

List of Figures ...... v

List of Tables...... vi

1. INTRODUCTION...... 1-1 1.1 Background ...... 1-1 1.2 Watershed Description ...... 1-1

2. METEOROLOGY DATA...... 2-1 2.1 Air Temperature ...... 2-1 2.2 Precipitation ...... 2-3

3. HYDROLOGY DATA...... 3-1 3.1 Hydrologic Monitoring...... 3-1 3.1.1 Historic Monitoring...... 3-1 3.1.2 Present Monitoring...... 3-1 3.2 Rating Curves...... 3-5 3.2.1 Weir Equations...... 3-5 3.2.2 Stage-Discharge Relationships ...... 3-5 3.3 Hydraulic Controls ...... 3-8 3.4 Physical Limnology...... 3-9 3.5 Taltson Flow Model ...... 3-10 3.6 Operating Procedure...... 3-11 3.6.1 Twin Gorges...... 3-11 3.6.2 Nonacho Lake ...... 3-11 3.7 South Valley Spillway (Trudel Creek)...... 3-22 3.8 Aerial Reconnaissance ...... 3-24

4. ICE PROCESSES...... 4-1 4.1 Water Temperature Data ...... 4-1

5. HYDROLOGICAL ISSUES OF THE WEMP...... 5-1 5.1 Water Levels and Ice Cover Upstream of Nonacho Dam ...... 5-1 5.1.1 Extent of Flooding ...... 5-1 5.1.2 Outflows from Nonacho Lake...... 5-1 5.1.3 Traditional and Potential Travel Routes ...... 5-9 5.1.4 Water Temperatures on Taltson River Downstream of Nonacho Lake ...... 5-9 5.2 Ice-related Water Level Fluctuations ...... 5-9

Northwest Territories Power Corporation - iii - Rescan™ Environmental Services Ltd. TABLE OF CONTENTS

5.2.1 Freeze-up Processes on Trudel Creek downstream of Twin Gorges ...... 5-9 5.2.2 Mid-winter Ice Conditions and Overflow Patterns ...... 5-9 5.2.3 Water Temperatures on the Taltson River below Twin Gorges...... 5-10 5.2.4 Break-up Processes on Trudel Creek and Downstream of Twin Gorges...... 5-10 5.3 Atypical Water Level Fluctuations...... 5-10 5.3.1 Outflows from Nonacho Lake...... 5-10 5.3.2 Water Level Fluctuations Between Nonacho Lake and Twin Gorges Forebay ...... 5-10 5.3.3 Water Level Fluctuations Along Trudel Creek...... 5-10 5.3.4 Water Level Fluctuations below Twin Gorges ...... 5-11 5.3.5 Variability of Outflows from Tsu Lake ...... 5-11 5.3.6 Effects of Dredging at the Mouth of the Taltson River...... 5-11 5.4 Sedimentation Due to Erosion...... 5-11 5.4.1 Geomorphic Characteristics...... 5-11 5.4.2 Extent of Bank Erosion and Channel Instabilities Along Waterway ...... 5-12 5.4.3 Extent of Sedimentation Along Trudel Creek...... 5-12 5.4.4 Suspended Sediment Transport Along Trudel Creek and the Taltson River 5-12

REFERENCES ...... R-1

Northwest Territories Power Corporation - iv - Rescan™ Environmental Services Ltd. TABLE OF CONTENTS

List of Figures

Figure Page

1.1-1 The Taltson River Drainage Basin ...... 1-2

1.1-2 Twin Gorges Hydro Plant on the Taltson River...... 1-3

2.1-1 Fort Smith Mean Annual Air Temperature (ºC) 1945 - 1999 ...... 2-4

3.2-1 Discharge Mass Curve from 1996 through 1997 for Water Survey of Canada 07QD007 and Twin Gorges Total Flow ...... 3-7

3.7-1 Average Monthly Flow with 95% Confidence Interval Entering Trudel Creek from the South Valley Spillway, 1987 - 2000...... 3-23

3.8-1 Trudel Creek and Twin Gorges (1955) Prior to Hydro Dam Construction...... 3-25

3.8-2 Trudel Creek Prior to Construction of the South Valley Spillway (1955) ...... 3-26

3.8-3 Twin Gorges Hydro Dam (1965) ...... 3-27

3.8-4 South Valley Spillway (1965) ...... 3-28

3.8-5 Upper Section of Trudel Creek (1965) after Spillway Construction...... 3-29

4.1-1 Daily Maximum and Minimum Water Temperatures in the Taltson River System at Twin Gorges Forebay, May-October, 1985 ...... 4-2

5.1-1 Nonacho Lake...... 5-8

Northwest Territories Power Corporation - v - Rescan™ Environmental Services Ltd. TABLE OF CONTENTS

List of Tables

Table Page

2.1-1 Temperature Normals (°C) for Fort Smith Airport ...... 2-1

2.1-2 Fort Smith Mean Annual Temperature (1945 to 1999)...... 2-3

2.2-1 Precipitation Normals for Fort Smith Airport ...... 2-4

2.2-2 Snow-Water-Equivalent Data for Sites In or Near the Taltson Basin...... 2-4

3.1-1 Summary of Inactive Water Survey of Canada Gauges in the Taltson Basin...... 3-1

3.1-2 Flow Summary (m3/s) of WSC Hydrometric Station 07QA001, Tsu Lake Outlet ...... 3-2

3.1-3 Flow Summary (m3/s) of WSC Hydrometric Station 07QC003, Hill Island Lake Inlet ...... 3-2

3.1-4 Flow Summary (m3/s) of WSC Hydrometric Station 07QD003, Nonacho Lake Outlet...... 3-3

3.1-5 Flow Summary (m3/s) of WSC Hydrometric Station 07QD004, Taltson River above Porter Lake Outflow...... 3-3

3.1-6 Flow Summary (m3/s) of WSC Hydrometric Station 07QD005, Porter Lake Outflow above Taltson River ...... 3-4

3.1-7 Flow Summary (m3/s) of WSC Hydrometric Station 07QD006, Porter Lake Outflow ...... 3-4

3.1-8 Summary of Active Water Survey of Canada Gauges in the Taltson Basin ...... 3-5

3.1-9 Water Level Summary (m) of Water Survey of Canada Hydrometric Station 07QD002...... 3-6

3.1-10 Flow Summary (m3/s) of Water Survey of Canada Hydrometric Station 07QD007...... 3-6

3.2-1 Flows Determined by Metering at Various Taltson River Locations, May-June, 1985...... 3-8

3.3-1 Hydraulic Controls within the Taltson River Basin ...... 3-8

3.4-1 Morphometric Features of the Lakes of the Taltson River System...... 3-9

3.5-1 Simulated Mean Monthly Flows (cfs) in the Taltson River at Various Locations Over its Length for Natural Conditions (1977-1985)...... 3-10

3.5-2 Mean Monthly Taltson River Flows (cfs) Over its Length Under Present Conditions (1977-1985)...... 3-11

3.6-1 Summary of Twin Gorges Average Monthly Forebay Elevations (m), 1987 - 2000 ...... 3-12

Northwest Territories Power Corporation - vi - Rescan™ Environmental Services Ltd. TABLE OF CONTENTS

3.6-2 Summary of Twin Gorges Average Monthly Tailrace Elevations (m), 1987 - 2000 ...... 3-13

3.6-3 Summary of Twin Gorges Average Monthly Plant Flow (m3/s), 1987 - 2000...... 3-14

3.6-4 Summary of Twin Gorges Average Monthly Spillway Flow (m3/s), 1987 - 2000...... 3-15

3.6-5 Summary of Twin Gorges Average Monthly Total Flow (m3/s) (Plant and Spillway), 1987 - 2000...... 3-16

3.6-6 Summary of Nonacho Reservoir Average Monthly Water Levels (m), 1987 - 2000...... 3-17

3.6-7 Summary of Nonacho Reservoir Average Monthly Gate Flow (m3/s), 1987 - 2000 ...... 3-18

3.6-8 Summary of Nonacho Reservoir Average Monthly Spillway Flow (m3/s), 1987 - 2000...... 3-19

3.6-9 Summary of Nonacho Reservoir Average Monthly Gap Flow (m3/s) (Tronka Chua Gap), 1987 - 2000...... 3-20

3.6-10 Summary of Nonacho Reservoir Average Monthly Total Flow (m3/s) (Gate Flow, Spillway, Tronka Chua Gap, and leakage), 1987 - 2000...... 3-21

3.7-1 Flow Through Twin Gorges Hydro Plant and Over the South Valley Spillway, 1987-2000..... 3-22

4.1-1 Lake Water Temperature Data, 1973 ...... 4-1

4.1-2 Spot Temperatures and Ranges in Temperatures (Max./Min.) in the Taltson River, Spring and Fall, 1985...... 4-3

5.1-1 Monitoring Activities to Address Hydrologic Issues ...... 5-2

Northwest Territories Power Corporation - vii - Rescan™ Environmental Services Ltd. 1. INTRODUCTION

TM 1. INTRODUCTION

This report is a compilation of all available and relevant meteorology and hydrology data collected, recorded, and purchased throughout the life of the Taltson Hydro Project. The data are analyzed, summarized, and discussed in the main body of the report, while the raw data are available on the accompanying CD and the 1998 Hydat CD. The purpose of the report is to compile and summarize meteorology/hydrology data and to identify which components of the Project require further investigation in order to meet the goals of the Water Effects Monitoring Program (WEMP) (FSC Engineers & Architects et al., 1999).

1.1 Background

The Northwest Territories Power Corporation (NTPC), formerly the Northern Canada Power Commission (NCPC), developed the Taltson Hydro Project to supply electricity to the Pine Point Mine. The Twin Gorges Hydro Plant is located on the Taltson River, some 56 km northeast of Fort Smith, at approximately 60o 25o N latitude and 110o 24o W longitude. (Figure 1.1-1). The Twin Gorges Hydro Plant was built on the Taltson River at Twin Gorges (Figure 1.1-2), just north of Elsie Falls. Electrical power was first produced in September 1965 from a single Francis turbine that was used to generate up to 18 MW from a gross head of 100 feet. In 1968, a dam was constructed on Nonacho Lake to increase the live storage of the basin. The Nonacho Lake storage dam is located 215 km northeast of Fort Smith at 61o 40o N latitude and 109o 56o W longitude (Figure 1.1-1). Three gates and a spillway were installed to regulate water storage as required. In 1976, four 1 MW Ossberger turbine-generator sets were installed in an extension of the powerhouse at Twin Gorges to meet additional demand for power.

After the closure of the Pine Point Mine, the Taltson Hydro Project began supplying power to the communities of Fort Smith and Fort Resolution. In addition to these communities, NTPC has extended its Taltson clientele to include Fort Fitzgerald, Alberta, and Hay River and Salt River, Northwest Territories. The peak load on the system is now in the order of 12.0 MW. The capacity of the Twin Gorges plant is now 19.2 MW.

1.2 Watershed Description

The following description, which was based on various published reports and internal documents on the Taltson River Basin from 1973 to present, was adapted from Coyne (2000).

The Taltson River flows in various directions in its upper reaches and then northward in its lower reaches, finally emptying into Great Slave Lake about 55 km east of Fort Resolution (Figure 1.1-1). Its drainage area is about 65,000 km2, bounded by the Slave River, Lake Athabasca, and the headwaters of the Dubawnt River.

Northwest Territories Power Corporation 1 - 1 Rescan™ Environmental Services Ltd. Cdr No. a6168L Job No. 503-3 02/23/2001-09:30am Res_AV

Arctic Ocean

Alaska NWT Yukon Nunavut

B.C.

PROJECT LOCATION

FIGURE 1.1-1 POWERNORTHWEST TERRITORIES TM CORPORATION The Taltson River Drainage Basin Cdr No. a6169L Job No. 503-3 02/22/2001-10:30am Res_AP

FIGURE 1.1-2 POWERNORTHWEST TERRITORIES Twin Gorges Hydro Plant TM CORPORATION on the Taltson River

Fuente: Water Management Consultants, 1994 INTRODUCTION

The drainage basin is located almost entirely within the Canadian Shield region, with bedrock being mainly Pre-Cambrian. The basin is characterized by thinly spread glacial and postglacial drift. The Taltson River delta is an alluvial plain, which is actually a part of an old delta built up by the Slave River.

From its headwaters, the main stem of the Taltson River is about 650 km long and links a series of interconnected lakes, some of the larger ones being Gray, Hjalmar, Norman, Tronka Chua, Nonacho, Taltson, King, Lady Grey, Benna Thy, Tsu and Deskenatlata. The Thoa River flows into the Tazin River at Hill Island Lake. The Tazin River meets the Taltson River some 75 km below the outlet of Hill Island Lake. The other principal tributary of the Taltson is the Tethual River, which drains the southwestern corner of the basin, including the northeastern corner of Alberta.

The Tazin River joins the main stem of the Taltson River approximately 60 km upstream of the Twin Gorges forebay. The headwaters of the Tazin, located in the southern most portion of the Taltson River Basin, drain via the Tazin River and other smaller tributaries into Tazin Lake. A dam was constructed in the late 1930s on the natural outlet of Tazin Lake. Approximately 90 to 95% of Tazin Lake inflow is now diverted south into the Charlot River for power production and ends up in Lake Athabasca. Most of it then enters the Slave River and from there Great Slave Lake. Flow over the dam on Tazin Lake enters Hill Island Lake via Tazin River. Flow exiting Hill Island Lake is comprised of flow from Tazin River and Thoa River. The major contributions come from Thoa River. Flow from Hill Island Lake reaches the mainstem of the Taltson River above Twin Gorges.

The Taltson River flows can leave the Twin Gorges forebay in two ways: 1) water used for generation goes through the power plant into the natural channel and then over Elsie Falls; and 2) the excess water goes over the spillway, located several kilometers from the plant, enters the upper reaches of the Trudel Creek, flows through Gertrude Lake, then turns northwest to rejoin the Taltson River immediately below Elsie Falls. The 33 kilometers of main channel from Elsie Falls to Tsu Lake contains several rapids. Flow is restricted in a long, narrow gorge called Nende Chute just before it enters Tsu Lake. Tsu Lake receives additional runoff from the Konth River, which drains the area to the northeast of Twin Gorges.

Outflow from Tsu Lake travels 18 km before meeting the Tethul River, which drains the southwestern portion of the Taltson River Basin. From this point on, the bed material of the Taltson River changes from non-erodible bedrock to erodible material deposited at one time by the Slave River. Bedrock outcrops occur further downstream at various rapids. A further 30 km downstream, the river enters Deskenatlata Lake where it picks up the drainage of the Rutledge River to the east. The Taltson leaves Deskenatlata Lake almost at the point it enters and then completes the 84 km final leg of its journey northward to Great Slave Lake.

Northwest Territories Power Corporation 1 - 4 Rescan™ Environmental Services Ltd. 2. METEOROLOGY DATA

TM 2. METEOROLOGY DATA

The primary meteorological parameters used to describe the Taltson site are air temperature and precipitation, both of which have a direct influence on the operation of the Taltson Hydro Project. Air temperatures dictate the timing for freeze-up and break-up, and precipitation affects the volume of flow available for power generation. The nearest meteorological station with a long-term period of record is Fort Smith Airport. Meteorological data from Fort Smith Airport are considered representative for the Taltson area and the available air temperature and precipitation records are summarized below.

2.1 Air Temperature

The monthly minimum, mean and maximum air temperatures for the Fort Simpson Airport meteorological station are summarized in Table 2.1-1 for the available period of record (1943 to 1999). The mean annual temperatures correlate well with data compiled by Environment Canada in their 1961 to 1990 Canadian Climate Normals for Fort Smith Airport.

The mean annual temperatures for Fort Smith appear in Table 2.1-2. It was not possible to generate a mean annual temperature for 1943, 1944 and 1947 due to missing data. Figure 2.1-1 shows the overall trend for mean annual air temperatures at Fort Smith. The overall warming trend is approximately +2.2°C over the 54 year period of record. This is consistent with Environment Canada observations that indicate the Mackenzie District air temperatures have increased 1.9°C over a 53 year period of record. The Mackenzie District has had six of the 10 warmest years in the past 10 years (1993, 1995, 1997, 1998, 1999 and 2000).

Table 2.1-1 Temperature Normals (°C) for Fort Smith Airport

Month Daily Minimum1 Daily Mean1 Daily Maximum1 January -30.5 -25.5 -20.5 February -27.2 -21.0 -15.3 March -20.5 -13.5 -6.5 April - 8.1 -1.8 4.6 May 1.3 7.8 14.4 June 7.0 13.7 20.4 July 9.6 16.2 22.8 August 7.9 14.4 20.8 September 2.5 7.9 13.1 October -3.8 0.3 4.4 November -15.6 -11.6 -7.7 December -25.6 -21.1 -16.6 Annual1 -8.6 -2.9 2.8 Annual2 -8.8 -3.0 2.7

1. Period of record was 1943 to 1999. 2. Period of record was 1943 to 1990. Report in Environment Canada – Atmospheric Environment Service – Canadian Climate Normals 1961 – 90.

Northwest Territories Power Corporation 2 - 1 Rescan™ Environmental Services Ltd. Cdr No. a6170f Job No. 503-3 02/22/2001-11:30am Res_AV

2.00

1.00 Note: 1998 warmest year on record for Mackenzie Valley District; according to Environment Canada 0.00 1945 1947 1949 1951 1953 1955 1957 1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999

-1.00

-2.00

-3.00

Mean Annual Air Temperature (°C) Mean Annual Air Temperature -4.00

-5.00

-6.00 Year Note: The Mackenzie District has the largest positive trend (+1.9°C) for increasing annual temperatures over a 53 year period of record for Canada. The District has had 6 of the 10 warmest years in the last 10 years (1998, 1993, 1999, 1997, 2000 and 1995) FIGURE 2.1-1 POWERNORTHWEST TERRITORIES TM CORPORATION Fort Smith Mean Annual Air Temperature (°C) 1945 - 1999

Source: Envirocon, 1986. METEOROLOGY DATA

Table 2.1-2 Fort Smith Mean Annual Temperature (1945 to 1999)

Year Temp. (°C) Comments Year Temp. (°C) Comments 1943 n/a Only have data for Sep. and Nov. 1971 -2.9 1944 n/a Missing January 1972 -4.8 1945 -3.8 1973 -1.7 1946 -3.8 1974 -3.6 1947 n/a Missing December 1975 -3.4 1948 -3.3 1976 -2.4 1949 -3.3 1977 -1.9 1950 -4.8 1978 -2.8 1951 -4.9 1979 -3.1 1952 -2.2 1980 -1.7 1953 -1.9 1981 -0.3 1954 -3.3 1982 -4.9 1955 -4.4 1983 -2.7 1956 -3.9 1984 -2.6 1957 -4.0 1985 -3.3 1958 -2.1 1986 -2.3 1959 -3.8 1987 -0.1 1960 -3.1 1989 -3.3 1961 -4.3 1990 -3.5 1962 -3.7 1991 -2.3 1963 -3.1 1992 -2.1 1964 -3.8 1993 -1.1 1965 -4.2 1994 -2.2 1966 -4.8 1995 -2.1 1967 -3.6 1996 -3.2 1968 -3.4 1997 -1.1 1969 -2.6 1998 0.9 1970 -3.0 1999 0.0

2.2 Precipitation

Table 2.2-1 summarizes the monthly precipitation at Fort Smith Airport for the available period of record (1943 to 1999). The values compare well with those compiled by Environment Canada and reported in the Canadian Climate Normals for 1951 to 1990. On an annual basis, rainfall comprises 66% of the total annual precipitation and snow- water-equivalent (SWE) contributes 34%. The month with the highest rainfall is July and the month with the highest snowfall is November.

Table 2.2-2 summarizes SWE data for basins near the Taltson site. SWE depends on snow depth and density (i.e., water content). For most NWT locations, snow density is approximately 10 to 15%, meaning that every 10 cm of accumulated snow contains 10 to 15 mm of SWE. SWE is a parameter often used by hydrologists because it describes the amount of potential runoff contained in the snow pack. SWE data is collected by monitoring approximately 10 sampling points at a particular site. Cores are collected from each sample point and their depth and density are measured. The SWE values from

Northwest Territories Power Corporation 2 - 3 Rescan™ Environmental Services Ltd. METEOROLOGY DATA

the 10 samples points are then averaged and the results added to the Environment Canada database.

Table 2.2-1 Precipitation Normals for Fort Smith Airport Rainfall1 Snowfall1 Total Precipitation2 Mean Snow Depth3 Month (mm) (cm) (mm) (cm) January 0.0 22.5 19.9 40 February 0.1 18.5 14.3 50 March 0.2 15.7 13.9 47 April 4.6 11.7 13.5 22 May 23.1 4.3 29.2 0 June 43.0 0.1 45.3 0 July 54.8 0.0 56.8 0 August 45.0 0.1 49.1 0 September 37.8 1.8 38.5 0 October 12.4 18.3 28.1 2 November 1.3 30.6 25.2 15 December 0.3 25.1 19.2 29 Annual1 222.6 148.8 n/a n/a Annual2 231.2 153.7 352.9 n/a Notes: n/a = not applicable. 1. Period of record was 1943 to 1999. 2. Period of record was 1943 to 1990. Source: Environment Canada – Atmospheric Environment Service – Canadian Climate Normals 1961 – 90. 3. Period of record was 1955 to 1999.

Table 2.2-2 Snow-Water-Equivalent Data for Sites In or Near the Taltson Basin Coordinates Elevation #years Mean Site Number Site Name Longitude Latitude (masl) record SWE (mm) 07QA-SC04 Piers Lake -111.17 60.32 260 18 106 07QA-SC01 Tortuous Lake -111.7 60.75 230 32 81 07QC-SC01 Dunvegan Lake -107.28 62.33 490 34 108 07QC-SC02 Whirlwind Lake -108.68 60.25 430 33 95 07QC-SC03 Alcantara Lake -108.28 60.9 425 32 96 07QC-SC04 Hill Island Lake -109.9 60.5 330 34 92 07QD-SC01 Thekulthili Lake -110.23 60.97 320 33 84 07QD-SC02 Nonacho Lake -109.67 61.72 320 34 102 07QD-SC03 Halliday Lake -109.03 61.38 350 34 95 07QD-SC04 Gray Lake -108.3 61.8 320 34 101 07QD-SC05 Dymond Lake -106.28 61.38 395 33 112

Note: SWE = snow-water-equivalent. It is the equivalent precipitation available from the melting of snow. This number depends upon the snow depth and density. For most northern locations, the snow density is roughly 1 mm of snow-water-equivalent for every 1 cm of snow accumulation.

Northwest Territories Power Corporation 2 - 4 Rescan™ Environmental Services Ltd. 3. HYDROLOGY DATA

TM 3. HYDROLOGY DATA

All available hydrological data relevant to the WEMP is summarized in the following section. The majority of the data can be found in previous reports on the Taltson Hydro Project (Envirocon, 1973, 1975, and 1986). The remaining data was obtained through NTPC.

3.1 Hydrologic Monitoring

3.1.1 Historic Monitoring

Within the Taltson Basin, six hydrometric stations were operated by the Water Survey of Canada (WSC) during the life of the Taltson Hydro Project (Table 3.1-1). Data collected at these sites were purchased by NTPC from WSC and are summarized in Tables 3.1-2 to 3.1-7.

Table 3.1-1 Summary of Inactive Water Survey of Canada Gauges in the Taltson Basin

Description of Period of Drainage Area Station ID Latitude Longitude Location Record (km2) 07QA001 60o28’1 N 111o30’46 W At outlet of Tsu 1952-1997 58,700 Lake 07QC003 60o30’18 N 109o38’56 W Near inlet to Hill 1968-1995 8,830 Island Lake 07QD003 61o39’36 N 109o58’7 W Near outlet of 1975-1977 22,600 Nonacho Lake 07QD004 61o52’32 N 107o40’12 W Taltson River above 1977-1990 9,660 Porter Lake outflow 07QD005 61o53’0 N 107o41’50 W Porter Lake Outflow 1971-1981 2,060 above Taltson River 07QD006 61o48’57 N 107o52’11 W Porter Lake outflow 1983-1990 2,050

3.1.2 Present Monitoring

Currently, there are only two hydrometric stations in operation (Table 3.1-8). One monitors lake levels on Nonacho Lake (07QD002) and the other monitors flow on the Taltson River below the hydro dam (07QD007). Water level data from Nonacho Lake are used to compute discharge through the underwater gates, over the spillway and through the Tronka Chua Gap. The station is now run by NTPC to determine discharge at various outflow locations on Nonacho Lake. Data collected at these sites were purchased by NTPC from WSC and are summarized in Tables 3.1-9 and 3.1-10.

Northwest Territories Power Corporation 3 - 1 Rescan™ Environmental Services Ltd. Table 3.1-2 Flow Summary (m3/s) of WSC Hydrometric Station 07QA001, Tsu Lake Outlet

Statistic January February March April May June July August September October November December Annual Average Flow 164 140 118 107 160 230 254 237 211 201 202 189 185 Daily Maximum 323 254 207 225 447 513 533 543 531 466 419 382 543 Daily Minimum 82.1 70.5 48.1 42.8 59.2 72.2 63.1 53.5 46.2 51 69.7 79.9 42.8

Table 3.1-3 Flow Summary (m3/s) of WSC Hydrometric Station 07QC003, Hill Island Lake Inlet

Statistic January February March April May June July August September October November December Annual Average Flow 22.3 17 13.7 14.3 73.9 83.1 59.1 47.6 48.5 53.2 44.5 32.3 42.7 Daily Maximum 51 35.4 27.4 86.5 253 264 132 133 147 167 137 78.4 264 Daily Minimum 8.1 6.23 5.86 5.92 7.36 31.1 19.7 14.3 13.5 13.5 11 9.57 5.86 Table 3.1-4 Flow Summary (m3/s) of WSC Hydrometric Station 07QD003, Nonacho Lake Outlet

Statistic January February March April May June July August September October November December Annual Average Flow 84.8 73.8 76.4 80.4 103 108 109 112 109 110 118 98.6 100 Daily Maximum 114 95.4 85.5 108 146 177 156 145 140 191 185 159 191 Daily Minimum 59.5 57.5 58.3 70.8 73.9 66 65.7 85 88.6 76.2 82.1 70.5 57.5

Table 3.1-5 Flow Summary (m3/s) of WSC Hydrometric Station 07QD004, Taltson River above Porter Lake Outflow

Statistic January February March April May June July August September October November December Annual Average Flow 26.6 21.4 18.3 18.9 60.5 114 81.9 65.5 63.5 58.4 46.9 35.6 51.5 Daily Maximum 45.6 33.4 27.2 73.1 360 362 187 184 123 106 83.8 62 362 Daily Minimum 15.1 11.6 10.8 9.91 9.77 34.7 23.7 23 27.7 24.6 21.3 18.3 9.77 Table 3.1-6 Flow Summary (m3/s) of WSC Hydrometric Station 07QD005, Porter Lake Outflow above Taltson River

Statistic January February March April May June July August September October November December Annual Average Flow 3.42 2.95 2.54 2.53 4.97 8.76 9.83 8.67 7.49 6.54 5.6 4.49 6.1 Daily Maximum 3.93 3.66 2.81 3.3 8.24 14.3 20 17.7 15 12.4 9.4 6.44 20 Daily Minimum 2.88 2.57 2.1 2 2.38 4.64 4.03 3.63 4 4.12 4.09 3.68 2

Table 3.1-7 Flow Summary (m3/s) of WSC Hydrometric Station 07QD006, Porter Lake Outflow

Statistic January February March April May June July August September October November December Annual Average Flow 6.51 5.1 4.49 4.05 8.19 16.3 17.1 15.2 13.6 12.2 10.7 8.81 10.2 Daily Maximum 9.2 7.2 7.4 6.86 28.1 36.1 47.8 46.6 30 21.2 16.2 14.6 47.8 Daily Minimum 2.09 1.97 1.72 1.68 1.82 6.84 10.9 7.69 5.19 3.71 2.95 2.46 1.68 HYDROLOGY DATA

Table 3.1-8 Summary of Active Water Survey of Canada Gauges in the Taltson Basin

Station ID Latitude Longitude Description of Location Period of Record 07QD002 61o43’50 N 109o40’15 W Nonacho Lake near 1962-present Lutselk’E (snowdrift) 07QD007 60o28’1 N 111o30’46 W Taltson River below 1994-present hydro dam

3.2 Rating Curves

3.2.1 Weir Equations

There are five sections in the Taltson Basin where the flow is currently computed from weir equations. A weir equation can be used to estimate discharge if the water passing over or through it drops through a critical depth (Prasuhn, 1987). Weir equations require upstream water level records to compute discharge through the weir. On Nonacho, an automated WSC hydrometric station (07QD002) records water levels so that the discharge through the gates, over the spillway, and through Tronka Chua Gap can be computed.

At Twin Gorges, forebay water levels are recorded by NTPC so that the discharge through the plant and over the South Valley Spillway can be computed. Historically, there has been some discrepancy between the summation of flow from the Twin Gorges Plant and the South Valley Spillway, and the flow recorded a short distance downstream on the Taltson River by WSC (07QD007). Figure 3.2-1 shows the summation of flow from the Twin Gorges plant and the South Valley Spillway, and the recorded flow downstream of the Taltson River. There are contributions to flow at WSC 07QD007 from the drainage area between the two monitoring locations that accounts for some of the discrepancy. However, the differences suggest that the rating curves require further investigation.

3.2.2 Stage-Discharge Relationships

Stage-discharge relationships are developed through flow monitoring and stage recording. Envirocon (1986) monitored flow and stage at Nonacho Dam, Tronka Chua Gap, and Hill Island Lake. The results are presented in Table 3.2-1.

Northwest Territories Power Corporation 3 - 5 Rescan™ Environmental Services Ltd. Table 3.1-9 Water Level Summary (m) of Water Survey of Canada Hydrometric Station 07QD002

Statistic January February March April May June July August September October November December Annual Average Daily Level 5.801 5.679 5.428 5.3 5.392 5.919 5.987 5.924 5.917 5.901 5.941 5.814 5.752 Daily Maximum 6.419 6.335 6.232 6.163 6.691 6.922 6.968 6.968 6.84 6.612 6.524 6.501 6.968 Daily Minimum 3.371 3.246 3.133 3.085 3.066 3.35 3.898 3.917 3.7 3.581 3.542 3.456 3.066

Table 3.1-10 Flow Summary (m3/s) of Water Survey of Canada Hydrometric Station 07QD007

Statistic January February March April May June July August September October November December Annual Average Flow 233 163 134 119 153 223 253 235 204 212 228 258 207 Daily Maximum 455 245 172 197 297 342 386 359 286 422 446 579 579 Daily Minimum 141 119 77.4 63.2 72.3 105 141 162 140 125 118 114 63.2 Cdr No. a6166L Job No. 503-3 02/21/2001-09:15am Res_AV

2.E+10

Twin Gorges Total 1.E+10 WSC 07QD007

1.E+10 3

1.E+10

8.E+09

6.E+09 Cummulative Volume (mCummulative Volume )

4.E+09

2.E+09

0.E+00 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 1996 1997 Date Notes: Error bars represent standard error of the mean.

FIGURE 3.2-1 POWERNORTHWEST TERRITORIES Discharge Mass Curve from 1996 through 1997 for TM CORPORATION Water Survey of Canada 07QD007 and Twin Gorges Total Flow HYDROLOGY DATA

Table 3.2-1 Flows Determined by Metering at Various Taltson River Locations, May-June, 1985

Lake Elevation Flow 3 Location Date (m) (ft) (m /s) 1. Nonacho Dam - spillway May 24 320.81 1052.50 76.91 - spillway + dam leakage May 23 320.81 1052.50 87.29 - spillway + dam leakage + one gate May 23 320.81 1052.50 103.1 - spillway Sept. 12 320.74 1052.28 78.36 - spillway + dam leakage Sept. 12 320.74 1052.28 94.77 - spillway + dam leakage + one gate Sept. 12 320.74 1052.28 99.61 - spillway + dam leakage + three gates Sept. 13 320.74 1052.28 148.35

2. Tronka Chua Gap May 31 321.00 1053.13 21.79 Sept. 14 320.74 1052.28 10.97

3. Hill Island Lake May 25 99.34 325.91 180.47 Sept. 11 97.73* 320.63* 56.50

* Relative to a local bench mark on the left bank of the river at the lake outlet = 100.00 metres. (Source: Envirocon, 1986) 3.3 Hydraulic Controls

There are a number of natural hydraulic controls on the Taltson River. A hydraulic control is a physical feature of the river channel that restricts the downstream passage of water. Most natural hydraulic controls on the Taltson River are waterfalls and rapids. Both are associated with an upsloping bed profile that leads to a crest, followed by a sharp decrease in bed elevation. Table 3.3-1 presents both natural and man-made hydraulic controls.

Table 3.3-1 Hydraulic Controls within the Taltson River Basin

Hydraulic Controls Location Description Nonacho Dam and Spillway Outlet of Nonacho Lake rock-fill dam Rapids Outlet of King Lake Five rock chutes around islands Rapids and Falls Outlet of Lady Grey Lake Several small falls and rapids Rapids Between Lady Grey and Benna Thy Lakes Rock chute about 4 metres Falls and Rapids Above Benna Thy Lake Large rapids and falls with over 20 m drop Falls 2-3 miles below Benna Thy Lake Vertical drop of 2-3 metres Natalkai Falls Just above Kozo Lake and Tazin River Large rapids and falls confluence Othikethe Falls In lower Tazin River Large rock chute Napie Falls Just below Kozo Lake Large falls of about 8 metres Falls At inflow to Methleka Lake Large falls of about 4-5 metres

(continued)

Northwest Territories Power Corporation 3 - 8 Rescan™ Environmental Services Ltd. HYDROLOGY DATA

Table 3.3-1 Hydraulic Controls within the Taltson River Basin (Completed)

Hydraulic Controls Location Description Three Bears Rapids At outlet of Methleka Lake Large rapids with about 10 m drop Naili Falls Just below Three Bears Rapids at Head of Large falls with drop of 6-8 metres Twin Gorges Reservoir Twin Gorges Dam At Twin Gorges Rock-fall dam and power house Elsie Falls Just below Twin Gorges 4-5 metre drop Natla Rapids About 10 miles below Elsie Falls Several rapids with total drop of about 8 metres Nende Rapids Just above Tsu Lake Very large rock chute with total drop of 30-40 metres Falls Just below Tsu Lake Two sets of falls with 2-3 metre drop Falls At Taltson/Tethul confluence Three sets of falls with about 5 m drop Rapids Just below Deskenatlata Lake Narrow fast chute with 2 metre drop Iche Falls, Rapids About 10 miles below Deskenatlata Lake Large (5 m) falls and several sets of rapids Oracha Falls About 2 miles below Big Island Large falls with drop of about 5 metres “Fishing Hole Rapids” Just above Rat River Large rapids with 5-6 m drop

(Source: Envirocon, 1986)

3.4 Physical Limnology

The morphometric features of lakes within the Taltson River System are presented in Table 3.4-1. Data was collected by Envirocon (1986).

Table 3.4-1 Morphometric Features of the Lakes of the Taltson River System

Max. Depth Mean Depth Area Volume Rank by Lake (Ft.) (m) (Ft.) (m) (acres) (acre-ft.) Volume Gray 164 (50) 43 (13) 18,000 777,000 4 Nonacho 289+ (88+) 39 (12) 143,000 5,560,000 1 Hjalmar 80 (35) 27 (8) 25,500 697,000 5 Taltson 148 (65) 53 (16) 18,500 977,000 3 King 155 (68) 50 (15) 6,000 298,000 8 Lady Grey 169 (74) 46 (14) 14,000 638,000 6 Benna Thy 98 (43) 32 (10) 3,500 113,000 9 Kozo 109 (48) 35 (11) 1,100 39,000 10½ Methleka 66 (29) 21 (6) 1,900 39,000 10½ Tsu 112 (49) 52 (16) 28,000 1,450,000 2 Deskenatlata 109 (48) 44 (13) 10,500 466,000 7

(Source: Envirocon, 1986)

Northwest Territories Power Corporation 3 - 9 Rescan™ Environmental Services Ltd. HYDROLOGY DATA

3.5 Taltson Flow Model

The following section summarizes the output data from the Taltson Flow Model (Froelich, 1988). The raw data are provided in Excel format on the CD accompanying this report.

The Taltson Flow Model (Froelich, 1988) was developed in 1985 to forecast water availability in the controlled watershed and describe the hydraulic regime in watercourses affected by the storage and release of water. The model was used to reconstruct historical flow data (1977 - 1985) from 17 locations within the Taltson River Basin. The model was also used to calculate water levels and flows for the same time period and locations under natural conditions (i.e. without the Twin Gorges Plant and the Nonacho Dam in place). Table 3.5-1 and 3.5-2 present the results from the model for both natural and operational conditions from 1977 to 1985.

Table 3.5-1 Simulated Mean Monthly Flows (cfs) in the Taltson River at Various Locations Over its Length for Natural Conditions (1977-1985)

Location* Nonacho Taltson King Lady Grey Benna Kozo Twin Tsu Deskenatlata Taltson Month Lake Lake Lake Lake Thy Lake Lake Gorges Lake Lake Bay Jan. 69.8 75.9 76.5 85.7 86.8 127.8 129.3 141.1 149.7 150.6 Feb. 60.3 67.1 68.6 78.6 76.8 110.9 110.7 120.8 128.3 128.9 Mar. 51.1 58.0 59.2 68.9 70.5 95.9 96.4 105.5 111.3 111.9 Apr. 41.8 48.2 49.6 59.2 60.6 88.3 88.7 94.0 101.6 102.7 May 50.7 49.3 50.6 61.5 64.7 144.9 150.4 146.7 163.3 166.5 Jun. 91.9 74.0 72.4 76.4 77.6 196.4 196.7 197.5 211.8 214.3 Jul. 118.2 106.2 105.5 109.2 109.9 205.7 204.5 214.9 227.8 229.6 Aug. 111.8 112.8 113.2 121.4 121.6 200.3 198.2 211.3 221.4 222.8 Sep. 102.1 105.8 106.8 115.7 116.9 181.5 184.5 196.2 205.4 207.6 Oct. 93.0 96.2 97.2 105.5 106.8 175.2 175.9 182.8 191.4 192.7 Nov. 90.0 92.2 93.6 101.3 102.7 172.4 168.9 183.2 194.2 195.8 Dec. 83.5 86.2 58.5 93.7 95.0 153.0 154.0 166.5 177.5 178.9

*At the lake outlet. (Source: Envirocon, 1986)

Rescan has revised the model at the request of NTPC. No changes were made to the input data or hydraulic/hydrologic assumptions used to run the model. One line of the model code was debugged in order to run the model. The major changes were to the output format. The model can now be run through Excel. A macro program was written in Excel that prompts the executable file of the model to run. Output data is automatically tabulated, plotted and summarized in one Excel file.

Northwest Territories Power Corporation 3 - 10 Rescan™ Environmental Services Ltd. HYDROLOGY DATA

Table 3.5-2 Mean Monthly Taltson River Flows (cfs) Over its Length Under Present Conditions (1977-1985)

Location* Nonacho Taltson King Lady Grey Benna Kozo Twin Tsu Deskenatlata Taltson Month Lake Lake Lake Lake Thy Lake Lake Gorges Lake Lake Bay Jan. 69.4 75.7 76.7 92.2 93.4 134.6 135.8 148.4 159.0 158.0 Feb. 63.2 67.8 70.6 84.8 85.8 116.9 113.7 127.1 133.8 134.4 Mar. 60.0 64.2 65.1 79.7 80.5 105.6 106.9 114.0 119.7 120.3 Apr. 56.8 59.6 61.0 74.3 75.6 103.4 104.1 107.4 114.2 116.0 May 58.3 60.7 62.4 78.6 81.6 168.8 170.9 163.2 181.6 154.5 Jun. 80.5 74.9 74.9 85.3 89.7 208.4 209.9 210.1 225.8 229.3 Jul. 97.3 90.9 90.9 104.9 107.3 202.9 199.5 217.1 228.9 233.9 Aug. 94.8 92.6 93.4 107.0 108.1 187.0 188.0 207.0 216.9 218.5 Sep. 80.7 83.9 84.7 99.3 100.4 170.8 171.6 180.4 190.0 191.7 Oct. 88.4 86.3 86.6 100.4 73.2 168.8 169.6 172.6 181.7 183.2 Nov. 88.5 88.3 91.3 108.4 111.3 184.8 194.6 198.6 210.1 213.3 Dec. 80.5 82.4 83.1 98.2 98.8 155.9 156.6 167.0 177.1 178.2

*At the lake outlet. (Source: Envirocon, 1986)

3.6 Operating Procedure

Operating data from Twin Gorges Plant and Nonacho Dam (1987-2000) are summarized below. The raw data are provided in Excel format on the CD accompanying this report.

3.6.1 Twin Gorges

Tables 3.6-1 to 3.6-5 summarize the operating conditions of the Twin Gorges Plant from 1987 to 2000. The Twin Gorges power plant consists of one Francis turbine and two functioning Ossberger turbines. Water sent through the Francis and/or Ossbergers turbines makes up plant flow. The South Valley Spillway releases all incoming flow not discharged through the plant. The spillway consists of a 200 m concrete structure situated about 13 km northeast of the plant. Both forebay and tailrace water levels were recorded by NTPC.

3.6.2 Nonacho Lake

Tables 3.6-6 to 3.6-10 summarize the operating conditions of Nonacho Reservoir from 1987 to 2000. Nonacho Lake was transformed into Nonacho Reservoir by the construction of a rockfill dam at its outlet in 1968. The dam contains three manually-controlled gates, which are periodically opened and closed as natural flows dictate. Flow through the gates is referred to as gate flow. Water levels on the reservoir are recorded daily. A spillway was constructed to convey flow downstream as water levels rise. A fraction of the spill flow exits from Nonacho Lake through a natural gap into Tronka Chua Lake. Flow from Nonacho Lake into Tronka Chua Lake via the natural gap is referred to as gap flow (Table 3.6-9). Leakage from the rockfill dam is typically around 10 m3/s.

Northwest Territories Power Corporation 3 - 11 Rescan™ Environmental Services Ltd. Table 3.6-1 Summary of Twin Gorges Average Monthly Forebay Elevations (m), 1987 - 2000

Year January February March April May June July August September October November December 1987 239.48 239.55 239.51 239.47 239.60 239.74 239.76 239.91 239.96 239.93 239.93 239.83 1988 239.75 239.76 239.68 239.58 239.62 239.95 233.28 240.28 240.20 240.13 240.03 239.95 1989 239.85 239.80 239.74 239.66 239.65 239.97 240.03 239.86 239.73 239.65 239.61 239.61 1990 239.63 239.63 239.57 239.50 239.57 239.62 239.69 239.73 239.71 239.73 239.73 239.74 1991 239.83 239.74 239.69 239.60 239.72 240.17 240.26 240.16 240.02 239.97 240.03 239.98 1992 239.89 239.87 239.77 239.67 239.96 240.22 240.21 240.08 239.98 239.88 239.92 239.98 1993 239.76 239.69 239.63 239.58 239.70 239.83 239.76 239.73 239.73 239.77 239.75 239.70 1994 239.75 239.69 239.58 239.51 239.77 239.97 239.89 239.77 239.66 239.59 239.56 239.53 1995 239.58 239.48 239.58 239.47 239.49 239.55 239.63 239.66 239.77 239.86 239.88 239.84 1996 239.82 239.87 239.71 239.62 239.64 239.66 239.73 240.03 239.85 239.72 239.71 239.66 1997 239.64 239.62 239.53 239.49 239.51 239.63 239.74 239.88 239.85 239.79 240.01 239.95 1998 239.94 239.79 239.69 239.63 239.80 239.94 239.88 239.68 239.60 239.53 239.52 239.52 1999 239.55 239.47 239.42 239.39 239.41 239.57 239.81 239.84 239.87 239.87 239.88 239.87 2000 239.87 239.79 239.72 239.63 239.73 239.83 239.86 239.77 239.78 239.95 240.05 239.99 Mean 239.74 239.70 239.63 239.56 239.65 239.83 239.39 239.88 239.84 239.81 239.83 239.80 Mean Max. 239.94 239.87 239.77 239.67 239.96 240.22 240.26 240.28 240.20 240.13 240.05 239.99 Mean Min. 239.48 239.47 239.42 239.39 239.41 239.55 233.28 239.66 239.60 239.53 239.52 239.52 Table 3.6-2 Summary of Twin Gorges Average Monthly Tailrace Elevations (m), 1987 - 2000

Year January February March April May June July August September October November December 1987 206.78 206.75 206.75 206.74 206.74 206.73 206.71 206.67 206.72 206.73 206.74 206.74 1988 208.60 208.70 208.74 208.25 208.00 208.09 207.88 208.01 208.06 208.11 208.28 208.33 1989 206.72 206.68 206.71 206.69 206.68 206.68 206.68 206.68 206.68 206.69 206.71 206.73 1990 206.72 206.73 206.71 206.69 206.68 206.69 206.69 206.69 206.69 206.69 206.71 206.71 1991 206.71 206.71 206.70 206.70 206.68 206.68 206.68 206.68 206.69 206.69 206.71 206.72 1992 206.72 206.71 206.70 206.70 206.69 206.65 206.68 206.68 206.69 206.69 206.88 206.77 1993 212.51 211.60 209.73 208.02 207.99 207.78 207.94 207.95 207.95 208.04 208.13 208.14 1994 208.26 208.23 208.11 208.15 207.99 207.86 207.96 207.95 207.96 208.01 208.05 208.18 1995 208.23 208.23 208.19 208.10 208.07 208.04 207.90 207.99 207.93 208.09 208.20 208.29 1996 208.26 208.17 208.19 208.10 208.03 208.03 208.02 208.00 208.04 208.12 208.20 208.28 1997 208.20 208.15 208.13 208.03 207.98 207.97 207.96 207.77 207.98 208.05 208.10 208.13 1998 207.70 208.18 208.11 208.00 207.97 207.70 207.85 207.95 207.96 208.00 208.09 208.16 1999 208.19 208.13 208.07 208.01 207.99 207.98 207.79 207.96 207.95 208.04 208.12 208.18 2000 208.20 208.14 208.09 208.02 207.99 207.97 207.65 207.93 207.98 208.03 208.10 208.16 Mean 207.99 207.94 207.78 207.59 207.53 207.49 207.46 207.49 207.52 207.57 207.64 207.68 Mean Max. 212.51 211.60 209.73 208.25 208.07 208.09 208.02 208.01 208.06 208.12 208.28 208.33 Mean Min. 206.71 206.68 206.70 206.69 206.68 206.65 206.68 206.67 206.68 206.69 206.71 206.71 Table 3.6-3 Summary of Twin Gorges Average Monthly Plant Flow (m3/s), 1987 - 2000

Annual Year January February March April May June July August September October November December Total 1987 79.0 59.3 59.2 55.5 56.5 53.1 45.1 32.2 46.1 50.7 55.5 57.2 649.4 1988 56.2 60.6 62.2 42.8 34.3 36.3 30.7 33.7 35.3 37.0 43.7 45.3 518.0 1989 45.8 42.7 41.8 37.0 32.0 31.7 32.9 33.3 31.6 37.2 45.2 50.7 462.0 1990 49.3 51.4 43.4 36.6 32.9 34.4 35.1 34.2 34.2 38.0 43.0 45.0 477.5 1991 44.3 42.9 40.8 38.0 32.2 33.6 32.7 32.7 33.6 37.3 43.5 45.8 457.3 1992 48.2 44.1 40.6 38.3 34.3 25.1 32.9 32.9 34.4 37.7 124.6 72.9 565.9 1993 43.4 43.2 37.6 35.7 34.9 22.2 32.9 33.5 34.9 36.2 38.8 41.0 434.2 1994 43.7 42.6 38.6 40.1 34.2 29.7 33.2 33.0 33.1 35.2 36.4 41.0 440.7 1995 41.5 41.3 39.8 36.3 34.9 34.1 29.7 32.4 30.8 35.9 40.3 43.9 440.8 1996 42.6 41.1 39.7 36.1 34.9 33.7 33.2 32.4 34.0 37.1 40.3 43.7 448.7 1997 43.6 41.8 40.7 36.8 34.1 34.0 33.7 20.8 34.1 36.7 38.7 39.6 434.6 1998 16.1 40.6 38.2 34.5 34.0 16.1 25.7 33.2 33.8 35.0 38.0 40.3 385.5 1999 41.1 39.2 37.3 35.2 34.7 34.2 23.4 33.5 32.6 35.9 38.7 40.7 426.5 2000 41.5 39.3 37.7 35.7 34.5 33.9 17.4 32.7 34.0 35.8 37.8 38.9 419.1 Mean 45.4 45.0 42.7 38.5 35.6 32.3 31.3 32.2 34.5 37.5 47.5 46.1 468.6 Mean Max. 79.0 60.6 62.2 55.5 56.5 53.1 45.1 34.2 46.1 50.7 124.6 72.9 649.4 Mean Min. 16.1 39.2 37.3 34.5 32.0 16.1 17.4 20.8 30.8 35.0 36.4 38.9 385.5 Table 3.6-4 Summary of Twin Gorges Average Monthly Spillway Flow (m3/s), 1987 - 2000

Annual Year January February March April May June July August September October November December Total 1987 37.9 56.3 46.7 35.4 73.5 120.0 127.8 200.2 216.6 202.2 204.8 159.2 1480.5 1988 125.2 129.3 100.2 66.3 79.4 213.4 374.7 392.8 344.3 305.1 252.4 204.1 2587.2 1989 168.0 147.1 119.8 93.3 91.9 227.0 255.2 173.5 118.6 91.2 76.0 76.6 1638.1 1990 84.0 83.6 62.7 41.3 63.4 79.9 105.6 117.0 111.1 117.0 117.0 121.0 1103.7 1991 158.4 121.7 106.0 74.5 115.8 331.0 380.1 325.5 249.2 225.5 254.0 230.8 2572.5 1992 183.7 178.2 133.3 97.6 223.6 357.7 350.4 278.8 227.0 180.8 199.5 230.8 2641.4 1993 150.7 118.6 93.2 74.7 122.9 182.9 151.4 136.4 135.0 155.0 147.2 123.4 1591.4 1994 145.6 120.9 77.7 52.1 157.0 259.6 212.8 155.5 106.4 81.4 67.1 59.6 1495.7 1995 75.5 42.8 76.3 42.7 45.7 65.5 94.1 109.7 156.3 197.3 207.1 188.6 1301.6 1996 179.8 205.6 127.1 91.0 99.7 106.3 137.4 293.4 195.3 133.3 127.4 107.7 1803.9 1997 94.4 87.5 58.3 45.6 51.7 92.4 135.4 201.1 188.3 160.9 274.2 239.7 1629.6 1998 235.6 160.5 114.8 91.1 164.0 235.6 202.8 110.6 81.0 57.2 52.8 54.4 1560.5 1999 63.28 37.18 23.59 17.56 23.09 72.29 170.07 185.42 199.86 197.33 201.48 199.82 1391.0 2000 198.4 157.9 128.9 92.7 132.5 180.0 193.6 150.8 153.6 239.6 292.2 262.2 2182.4 Mean 135.8 117.7 90.6 65.4 103.2 180.2 206.5 202.2 177.3 167.4 176.7 161.3 1784.2 Mean Max. 235.6 205.6 133.3 97.6 223.6 357.7 380.1 392.8 344.3 305.1 292.2 262.2 2641.4 Mean Min. 37.9 37.2 23.6 17.6 23.1 65.5 94.1 109.7 81.0 57.2 52.8 54.4 1103.7 Table 3.6-5 Summary of Twin Gorges Average Monthly Total Flow (m3/s) (Plant and Spillway), 1987 - 2000

Annual Year January February March April May June July August September October November December Total 1987 116.9 115.6 46.7 149.6 130.1 173.4 173.4 233.0 262.8 253.2 260.6 216.2 2131.5 1988 181.7 190.2 162.5 109.0 113.7 251.0 405.5 426.0 379.5 343.0 296.1 249.6 3108.0 1989 213.7 189.6 161.9 130.0 123.5 258.6 287.5 205.9 146.0 128.1 120.8 126.9 2092.6 1990 133.1 134.9 105.9 78.2 95.8 113.9 140.6 151.6 145.8 155.5 160.4 166.3 1582.0 1991 203.3 165.1 146.6 112.1 147.9 364.7 413.0 358.2 282.9 263.0 297.7 276.9 3031.4 1992 232.2 222.5 174.2 135.5 257.7 383.0 383.4 311.6 261.7 218.7 324.0 303.9 3208.5 1993 194.1 161.8 130.8 110.4 157.5 207.7 184.3 169.9 169.6 191.2 185.4 164.3 2027.0 1994 189.3 163.5 116.3 92.2 191.2 289.3 246.0 188.4 139.5 116.5 103.5 100.6 1936.4 1995 117.0 84.1 116.1 79.0 80.6 99.6 123.8 142.1 187.1 233.3 247.3 232.4 1742.4 1996 222.4 245.2 166.7 127.1 134.7 140.0 170.6 325.8 229.3 170.4 167.6 151.4 2251.2 1997 138.0 129.4 99.0 82.4 85.7 126.4 169.1 221.9 222.4 197.7 313.0 279.3 2064.1 1998 251.7 201.2 153.1 125.6 198.1 251.7 228.5 143.7 114.8 92.2 90.8 94.8 1946.2 1999 104.4 76.4 60.8 52.8 57.8 106.5 193.5 218.9 232.5 233.2 240.1 240.5 1817.4 2000 239.9 197.3 166.5 128.3 167.0 213.9 211.1 183.4 187.6 275.4 330.0 301.1 2601.5 Mean 181.3 162.6 129.1 108.0 138.7 212.8 237.9 234.3 211.5 205.1 224.1 207.4 2252.9 Mean Max. 251.7 245.2 174.2 149.6 257.7 383.0 413.0 426.0 379.5 343.0 330.0 303.9 3208.5 Mean Min. 104.4 76.4 46.7 52.8 57.8 99.6 123.8 142.1 114.8 92.2 90.8 94.8 1582.0 Table 3.6-6 Summary of Nonacho Reservoir Average Monthly Water Levels (m), 1987 - 2000

Year January February March April May June July August September October November December 1987 320.73 320.64 320.56 320.49 320.66 320.91 321.00 320.92 320.90 320.91 320.93 320.94 1988 320.91 320.78 320.67 320.58 320.56 321.22 321.55 321.54 321.35 321.20 321.13 321.07 1989 320.86 320.76 320.67 320.58 320.70 321.00 321.03 320.95 320.94 320.92 320.88 320.83 1990 320.68 320.62 320.56 320.49 320.44 320.64 320.77 320.78 320.84 320.87 320.83 320.77 1991 320.90 320.85 320.78 320.73 320.95 321.52 321.41 321.13 320.95 321.06 321.15 321.10 1992 320.97 320.90 320.83 320.76 320.84 321.31 321.36 321.09 320.95 320.87 320.84 320.82 1993 320.74 320.66 320.58 320.52 320.73 320.99 320.94 320.93 320.96 320.91 320.84 320.63 1994 320.75 320.67 320.62 320.55 320.65 321.07 321.08 320.88 320.72 320.63 320.60 320.59 1995 320.55 320.50 320.45 320.40 320.43 320.58 320.77 320.86 320.99 321.12 321.09 321.00 1996 320.90 320.79 320.71 320.64 320.68 321.09 321.11 320.90 320.78 320.69 320.65 320.62 1997 320.57 320.50 320.44 320.38 320.39 320.67 320.88 320.87 320.83 320.85 320.89 320.90 1998 320.82 320.73 320.64 320.58 320.89 321.02 320.86 320.75 320.73 320.77 320.80 320.79 1999 320.73 320.67 320.61 320.55 320.62 320.92 321.12 321.08 321.10 321.07 321.03 320.98 2000 320.88 320.76 320.64 320.55 320.76 320.95 320.91 320.83 321.00 321.15 321.17 321.08 Mean 320.8 320.7 320.6 320.6 320.7 321.0 321.1 321.0 320.9 320.9 320.9 320.9 Mean Max. 321.0 320.9 320.8 320.8 320.9 321.5 321.5 321.5 321.3 321.2 321.2 321.1 Mean Min. 320.6 320.5 320.4 320.4 320.4 320.6 320.8 320.8 320.7 320.6 320.6 320.6 Table 3.6-7 Summary of Nonacho Reservoir Average Monthly Gate Flow (m3/s), 1987 - 2000

Annual Year January February March April May June July August September October November December Total 1987 12.4 15.8 15.6 15.5 15.9 16.3 0.0 0.0 0.0 0.0 0.0 0.0 91.4 1988 8.5 16.1 15.9 15.7 15.6 25.4 53.5 53.4 52.2 44.7 16.9 16.8 334.7 1989 16.3 16.1 15.9 15.7 16.0 43.3 34.1 0.0 0.0 0.0 0.0 0.0 157.3 1990 0.0 0.0 0.0 13.0 15.5 16.0 16.3 12.1 0.0 0.0 0.0 0.0 72.9 1991 0.0 0.0 0.0 0.0 10.5 35.5 35.1 33.9 11.0 0.0 0.0 0.0 125.9 1992 0.0 0.0 0.0 0.0 21.7 34.6 34.8 33.6 24.8 16.3 16.3 16.2 198.5 1993 15.6 15.5 15.3 15.2 15.6 16.1 6.7 0.0 0.0 8.7 15.8 15.5 140.2 1994 15.7 15.5 15.4 15.3 15.5 16.3 16.3 15.9 15.6 15.4 15.4 15.4 187.6 1995 15.3 15.2 15.1 15.0 15.1 15.4 15.7 15.9 16.1 16.3 16.3 16.1 187.4 1996 15.9 16.3 15.6 15.5 15.5 35.9 48.9 39.7 15.7 15.5 16.0 15.4 266.0 1997 15.3 15.2 15.1 15.0 15.0 15.5 15.9 15.9 15.8 15.9 15.9 15.9 186.4 1998 15.8 15.6 15.5 15.3 15.9 32.3 15.9 15.7 7.8 0.0 0.0 0.0 149.8 1999 0.0 0.0 0.0 0.0 0.0 0.0 17.9 28.9 16.3 16.2 16.2 16.1 111.6 2000 15.9 15.7 15.5 15.3 15.7 16.0 16.0 15.8 16.1 16.4 16.4 16.3 191.1 Mean 10.5 11.2 11.1 11.9 14.5 22.8 23.4 20.1 13.7 11.8 10.4 10.3 171.5 Mean Max. 16.3 16.3 15.9 15.7 21.7 43.3 53.5 53.4 52.2 44.7 16.9 16.8 334.7 Mean Min. 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 72.9 Table 3.6-8 Summary of Nonacho Reservoir Average Monthly Spillway Flow (m3/s), 1987 - 2000

Annual Year January February March April May June July August September October November December Total 1987 55.5 44.3 34.9 28.5 47.2 79.8 94.4 81.8 78.4 80.7 83.0 85.0 793.3 1988 80.6 61.2 47.9 37.6 35.7 133.0 193.9 192.9 155.0 128.4 115.3 105.5 1287.1 1989 72.7 59.5 47.7 37.1 52.5 94.2 99.2 87.1 84.3 82.0 75.8 68.7 860.7 1990 59.9 51.6 44.2 35.9 31.8 54.5 71.5 73.7 81.5 87.2 80.1 72.4 744.3 1991 78.4 71.8 62.4 54.7 88.7 187.8 166.0 115.7 85.6 103.6 118.0 109.9 1242.7 1992 89.4 78.9 69.6 59.0 71.2 147.8 156.5 108.4 87.2 74.8 70.6 67.0 1080.4 1993 58.4 47.9 39.8 33.2 58.8 94.8 86.9 85.7 89.0 81.7 72.2 44.9 793.3 1994 59.8 49.8 43.2 36.5 47.9 108.3 109.1 77.8 55.5 44.4 41.6 40.2 714.1 1995 36.3 31.4 26.4 22.0 24.6 39.6 62.4 74.4 94.4 116.1 111.1 96.9 735.4 1996 80.7 65.5 54.1 45.8 51.3 110.7 114.0 80.6 63.6 51.9 47.2 43.5 809.0 1997 37.7 31.3 25.1 20.2 20.7 50.3 77.9 76.6 70.6 73.0 79.6 79.7 642.8 1998 69.6 57.2 46.5 39.1 80.1 99.6 74.2 59.9 56.9 61.6 65.6 65.1 775.4 1999 57.4 49.6 42.3 35.8 43.8 83.6 117.1 108.9 113.1 107.2 100.9 93.4 953.1 2000 77.1 60.4 46.3 36.4 61.9 88.5 82.0 70.5 96.1 122.2 126.4 110.3 978.1 Mean 65.3 54.3 45.0 37.3 51.2 98.0 107.5 92.4 86.5 86.8 84.8 77.3 886.4 Mean Max. 89.4 78.9 69.6 59.0 88.7 187.8 193.9 192.9 155.0 128.4 126.4 110.3 1287.1 Mean Min. 36.3 31.3 25.1 20.2 20.7 39.6 62.4 59.9 55.5 44.4 41.6 40.2 642.8 Table 3.6-9 Summary of Nonacho Reservoir Average Monthly Gap Flow (m3/s) (Tronka Chua Gap), 1987 - 2000

Annual Year January February March April May June July August September October November December Total 1987 8.5 6.0 4.0 2.7 6.7 14.1 22.6 14.5 13.7 14.3 14.8 15.3 137.2 1988 14.2 9.8 6.8 4.6 4.2 27.5 45.0 43.3 32.7 25.8 21.9 20.1 256.0 1989 12.4 9.4 6.8 4.5 7.9 17.5 18.7 15.8 15.1 14.6 13.1 11.5 147.1 1990 9.5 7.6 6.0 4.2 3.4 8.3 12.1 12.6 14.5 15.8 14.1 12.3 120.5 1991 13.7 12.3 10.1 8.3 16.4 42.7 35.9 22.7 15.4 19.7 23.2 21.2 241.5 1992 16.3 13.9 11.7 9.3 12.1 30.9 33.1 21.0 15.8 12.9 12.0 11.1 200.1 1993 9.0 6.8 5.2 3.8 9.1 16.7 15.0 14.7 15.4 13.9 11.9 6.2 127.6 1994 9.2 7.2 5.8 4.5 6.8 19.6 19.7 13.0 8.3 6.1 5.5 5.2 111.0 1995 4.5 3.5 2.5 1.7 2.2 5.1 9.8 12.3 16.6 21.2 20.2 17.1 116.5 1996 13.6 10.4 8.1 6.4 7.5 20.1 20.8 13.6 10.1 7.6 6.7 5.9 130.8 1997 4.7 3.5 2.3 1.3 1.4 7.3 13.1 12.8 11.5 12.0 13.4 13.4 96.8 1998 11.3 8.7 6.5 5.0 13.5 17.7 12.3 9.3 8.7 9.6 10.5 10.4 123.5 1999 8.8 7.1 5.7 4.3 6.0 14.3 21.5 19.7 20.6 19.3 18.0 16.4 161.6 2000 12.9 9.4 6.5 4.5 9.7 15.3 13.9 11.5 16.9 22.6 23.5 20.0 166.6 Mean 10.6 8.3 6.3 4.6 7.6 18.4 21.0 16.9 15.4 15.4 14.9 13.3 152.6 Mean Max. 16.3 13.9 11.7 9.3 16.4 42.7 45.0 43.3 32.7 25.8 23.5 21.2 256.0 Mean Min. 4.5 3.5 2.3 1.3 1.4 5.1 9.8 9.3 8.3 6.1 5.5 5.2 96.8 Table 3.6-10 Summary of Nonacho Reservoir Average Monthly Total Flow (m3/s) (Gate Flow, Spillway, Tronka Chua Gap, and leakage), 1987 - 2000

Annual Year January February March April May June July August September October November December Total 1987 86.1 75.6 63.8 82.3 79.3 109.6 122.5 106.7 102.5 105.3 108.3 110.7 1152.5 1988 113.6 97.0 80.2 67.1 64.8 197.4 304.7 303.2 351.9 210.3 165.9 153.3 2109.2 1989 111.6 94.8 80.0 66.6 86.0 166.0 162.7 113.4 109.9 107.0 99.1 90.2 1287.1 1990 79.3 68.9 59.6 62.3 59.8 88.5 110.1 108.7 106.4 113.4 104.5 94.9 1056.5 1991 102.4 94.2 82.4 72.7 126.0 278.5 249.1 183.4 122.4 134.2 152.4 142.1 1739.7 1992 116.3 103.2 91.4 78.0 115.2 225.0 236.4 173.9 138.3 114.2 108.9 104.3 1605.1 1993 92.4 79.4 69.2 61.1 92.9 137.6 118.4 110.2 114.3 114.1 109.5 75.7 1174.7 1994 94.2 81.6 73.5 65.2 79.3 154.3 155.2 116.4 88.1 75.0 71.5 69.8 1124.0 1995 65.0 58.8 52.6 47.1 50.4 69.0 97.3 112.2 137.1 163.9 157.7 140.1 1151.2 1996 120.0 101.7 87.1 76.8 83.5 176.9 194.0 143.6 98.9 84.4 79.1 73.8 1319.8 1997 66.7 58.8 51.1 44.9 45.5 82.3 116.6 115.0 107.5 110.5 118.6 118.9 1036.3 1998 106.2 90.9 77.6 68.4 119.3 159.7 112.0 94.2 82.7 80.7 85.5 85.0 1162.2 1999 75.5 65.9 57.0 49.0 58.9 107.6 166.8 167.7 160.3 152.9 145.2 135.8 1342.5 2000 115.6 94.8 77.4 65.1 96.7 129.7 121.6 107.4 139.2 171.5 176.8 156.8 1452.6 Mean 96.1 83.2 71.6 64.8 82.7 148.7 162.0 139.7 132.8 124.1 120.2 110.8 1336.7 Mean Max. 120.0 103.2 91.4 82.3 126.0 278.5 304.7 303.2 351.9 210.3 176.8 156.8 2109.2 Mean Min. 65.0 58.8 51.1 44.9 45.5 69.0 97.3 94.2 82.7 75.0 71.5 69.8 1036.3 HYDROLOGY DATA

3.7 South Valley Spillway (Trudel Creek)

The South Valley Spillway allows flow to pass through Trudel Creek, bypassing Twin Gorges Plant, and into the main stem of the Taltson River when the water level of the Twin Gorges Forebay exceeds the elevation of the spillway. The South Valley Spillway is a 200 m long overflow concrete spillway constructed along the former divide between the upper portion of the Taltson River Basin and the sub-basin of Trudel Creek. The spillway is approximately 13 km northeast of the plant. Water passed over the South Valley Spillway travels around 30 km through Trudel Creek, Trudel Lake, and Gertrude Lake before re-entering the main stem of the Taltson River approximately 2 km downstream of Twin Gorges Plant.

Before the construction of the Twin Gorges Plant and South Valley Spillway, the drainage area of Trudel Creek was relatively small. The location of the South Valley Spillway was formerly the headwater area of the Trudel Creek Drainage Basin. The current drainage basin includes the entire drainage area of the Taltson River upstream of the Twin Gorges Plant. The runoff from this area is divided between plant flow and overflow. The actual division of flow for a given year depends on power requirements. Table 3.7-1 presents the historic division of flow between the Twin Gorges Hydro Plant and the South Valley Spillway. Figure 3.7-1 shows the average monthly flow from the South Valley Spillway into Trudel Creek from 1987 to 2000.

Table 3.7-1 Flow Through Twin Gorges Hydro Plant and Over the South Valley Spillway, 1987-2000

South Valley Spillway Twin Gorges Hydro Plant Year Spillway Flow (m3/s) Percentage Plant Flow (m3/s) Percentage 1987 1480.5 69.5 649.4 30.5 1988 2587.2 83.3 518.0 16.7 1989 1638.1 78.0 462.0 22.0 1990 1103.7 69.8 477.5 30.2 1991 2572.5 84.9 457.3 15.1 1992 2641.4 82.3 565.9 17.6 1993 1591.4 78.5 434.2 21.4 1994 1495.7 77.2 440.7 22.8 1995 1301.6 74.7 440.8 25.3 1996 1803.9 80.1 448.7 19.9 1997 1629.6 78.9 434.6 21.1 1998 1560.5 80.2 385.5 19.8 1999 1391.0 76.5 426.5 23.5 2000 2182.4 83.9 419.1 16.1

Northwest Territories Power Corporation 3 - 22 Rescan™ Environmental Services Ltd. Cdr No. a6171f Job No. 503-3 02/23/2001-11:30am Res_AV

300

250

200

150 Flow (m³/s)

100

50

0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

FIGURE 3.7-1

NORTHWEST TERRITORIES Average Monthly Flow with 95% Confidence Interval Entering TM POWER CORPORATION Trudel Creek from the South Valley Spillway, 1987 - 2000

Source: Envirocon, 1986. HYDROLOGY DATA

3.8 Aerial Reconnaissance

Figures 3.8-1 to 3.8-5 show pre- and post-construction aerial photographs of Twin Gorges and Trudel Creek. Aerial reconnaissance and airlift flights were also undertaken by Envirocon (1986). The dates and locations are presented in the 1986 report. However, the photos were not available for this report.

Northwest Territories Power Corporation 3 - 24 Rescan™ Environmental Services Ltd. Cdr No. a6172f Job No. 503-3 02/22/2001-05:00pm Res_AP

TwinTwin GorgesGorges

TrudelTrudel CreekCreek

TaltsonTaltson RiverRiver

FIGURE 3.8-1 POWERNORTHWEST TERRITORIES Trudel Creek and Twin Gorges (1955) TM CORPORATION Prior to Hydro Dam Construction

Fuente: Water Management Consultants, 1994 Cdr No. a6173f Job No. 503-3 02/22/2001-05:00pm Res_AP

TrudelTrudel CreekCreek

FIGURE 3.8-2 POWERNORTHWEST TERRITORIES Trudel Creek Prior to Construction TM CORPORATION of the South Valley Spillway (1955)

Fuente: Water Management Consultants, 1994 Cdr No. a6174f Job No. 503-3 02/22/2001-05:00pm Res_AP

TwinTwin GorgesGorges

TrudelTrudel CreekCreek

TaltsonTaltson RiverRiver

FIGURE 3.8-3 POWERNORTHWEST TERRITORIES Twin Gorges Hydro Dam (1965) TM CORPORATION

Fuente: Water Management Consultants, 1994 Cdr No. a6175f Job No. 503-3 02/23/2001-10:00am Res_AP

SouthSouth ValleyValley SpillwaySpillway

TrudelTrudel CreekCreek

FIGURE 3.8-4 POWERNORTHWEST TERRITORIES South Valley Spillway (1965) TM CORPORATION

Fuente: Water Management Consultants, 1994 Cdr No. a6176f Job No. 503-3 02/23/2001-10:00am Res_AP

TrudelTrudel CreekCreek

FIGURE 3.8-5

NORTHWEST TERRITORIES Upper Section of Trudel Creek (1965) TM POWER CORPORATION after Spillway Construction

Fuente: Water Management Consultants, 1994 4. ICE PROCESSES

TM 4. ICE PROCESSES

No documented account of winter freeze-up and summer break-up of ice exists for the Taltson Basin. The general pattern of ice formation and melt is qualitatively discussed in Envirocon (1986). Surface waters of lakes in the Taltson Basin typically freeze in October and break-up in May of the following year. The northern portion of the basin freezes two to three weeks earlier than the southern portion. Observations as late as mid- June of 1985 revealed ice on Nonacho, Taltson, King and Lady Grey lakes.

4.1 Water Temperature Data

Envirocon (1975) collected water temperature data from the Taltson River in June-July and September, 1973. The depth of monitoring was not specified. Table 4.1-1 and 4.1-2 present the water temperature data from the Taltson River.

Table 4.1-1 Lake Water Temperature Data, 1973

Temperature (ºC) Location June-July 1973 September 1973 Nonacho Lake 12.2 – 20.0 10.0 – 12.0 Gray Lake 14.8 – 17.6 -- Hjalmar Lake -- 12.0 Taltson Lake 15.2 11.0 King Lake 16.0 – 17.0 12.0 Lady Grey Lake 17.6 – 19.0 -- Benna Thy Lake -- 18.8 Twin Gorges Reservoir -- 13.0 Tsu Lake -- 13.0 Kozo Lake 19.8 -- Methleka Lake 20.0 --

(Source: Envirocon, 1975)

Envirocon (1986) collected continuous water temperature data from the Twin Gorges forebay from May to October, 1995 (Figure 4.1-1). Water temperatures rose steadily to early August, then dropped off in the fall as expected. The raw data used to generate Figure 4.1-1 are not available.

Northwest Territories Power Corporation 4 - 1 Rescan™ Environmental Services Ltd. Cdr No. a6167L Job No. 503-3 02/21/2001-09:40am Res_AV

20

18

16 maximum

14

minimum

12

10

8 NOTE: recorded at 1 m depth using a recording thermograph.

Water Temperature (°C) Temperature Water maximum/minimum 6 thermometer gross range

Sept. 7 - Oct. 4, 1985 4

2

0 MAY JUNE JULY AUGUST SEPTEMBER OCTOBER

1985 FIGURE 4.1-1 POWERNORTHWEST TERRITORIES Daily Maximum and Minimum Water Temperatures in the TM CORPORATION Taltson River System at Twin Gorges Forebay, May-October, 1985

Source: Envirocon, 1986. ICE PROCESSES

Table 4.1-2 Spot Temperatures and Ranges in Temperatures (Max./Min.) in the Taltson River, Spring and Fall, 1985

Date Location Water Temp. (ºC) Spring: May 15 Below Nende Rapids* 2.2 May 18 Below Elsie Falls* 3.0 May 19 Slough above Natla Rapids 9.5 May 21 Slough above Nende Rapids 9.0 May 24 - 25 Above Deskenatlata Lake* 3.0 - 10.0 May 25 - 26 Above Deskenatlata lake* 2.5 - 10.0 May 28 - 29 Above Oracha Falls* 5.5 - 8.0 May 30 - 31 Below Nonacho Lake* 0.5 - 7.0 May 31 - June 1 Below Nonacho Lake* 2.0 - 6.5 June 1 - 2 Below Nonacho Lake* 2.0 - 7.0 June 3 - 4 In Benna Thy Lake 5.0 - 9.5 June 4 - 5 Above Methleka Lake 7.5 - 14.0 June 5 - 6 Above Methleka Lake 8.0 - 13.5

Fall: September 7 (Shutdown) Slough above Natla Rapids 9.5 September 7 (Shutdown) Below Elsie Falls* 11.5 September 8 (Open) Below Elsie Falls* 12.0 September 8 (Open) Slough above Natla rapids 11.0 September 9 Below Tethul Confluence* 11.5 September 10 Deskenatlata Lake 11.5 September 9 - 10 Above Deskenatlata Lake* 9.5 - 12.0 September 7 - October 4 Forebay - Twin Gorges* 2.5 - 9.0

*Mainstream riverine habitats.

Northwest Territories Power Corporation 4 - 3 Rescan™ Environmental Services Ltd. 5. HYDROLOGICAL ISSUES OF THE WEMP

TM 5. HYDROLOGICAL ISSUES OF THE WEMP

The following section identifies existing data and the data required to address the hydrologic issues outlined in the WEMP. This section is divided into four sub-sections to match those presented in Table 4 of the WEMP. Table 4 of the WEMP has been reproduced below as Table 5.1-1. The issues, components and text have not been modified.

5.1 Water Levels and Ice Cover Upstream of Nonacho Dam

5.1.1 Extent of Flooding

The dam on Nonacho Lake has caused the lake level to rise by approximately 2.0 m (Envirocon, 1975). Figure 5.1-1 shows pre and post dam water levels on Nonacho Lake. The differences between pre- and post-dam conditions are difficult to see due to the size of the lake. However, details of specific areas can be investigated with the aid of computer software (Autocad).

No map has been produced that shows the pre- and post-dam water levels on Hjalmar and Gray lakes. However, the data required to generate such a map can be obtained by use of the Taltson Flow Model and site investigations.

The Taltson Flow Model has generated historic water levels – pre- and post-construction of the Nonacho Dam – on Nonacho, Hjalmar and Gray lakes. Bathymetric investigations of Hjalmar and Gray lakes, along with data from the Taltson Flow Model, would provide the data necessary to quantify the degree of inundation around Hjalmar and Gray lakes.

Although site investigations have identified hydraulic controls on the Taltson River by visual inspection, the investigation covered the area downstream of Nonachho Dam. Therefore, no hydraulic controls have been identified between Nonacho and Hjalmar lakes and between Nonacho and Gray lakes.

The recommendation of Component 1.1 (Table 5.1-1) is to monitor daily inflows into Nonacho Lake from hydrometeorologic stations installed upstream. Hydrometeorologic stations at the outlets of Hjalmar and Gray lakes would aid in defining the hydrologic features of the area. However, these stations are not required to determine the extent of flooding around these lakes.

5.1.2 Outflows from Nonacho Lake

There are four distinct outflows from Nonacho Lake: gates, spillway, Tronka Chua Gap, and leakage. Discharge through the gated system, over the spillway, and through the Tronka Chua Gap is determined by weir equations. The assumptions of the weir equations should be verified for accuracy through manual flow monitoring.

Northwest Territories Power Corporation 5 - 1 Rescan™ Environmental Services Ltd. Table 5.1-1 Monitoring Activities to Address Hydrologic Issues

Activities Reconnaissance Level Reporting/Long Issue Component Preparatory Work Short Term Field Monitoring Schedule Survey Term Monitoring 1. Changes in 1.1 Extent of flooding 1. Assemble and compare pre 1. From either aircraft None 1.Monitor daily 1999/00 water levels and around lake. and post regulation maps and observations or analysis of inflows into Nonacho ice cover aerial photos to determine aerial photos categorize the Lake from characteristics on flooded area. degree of inundation around hydrometeorologic Nonacho Lake 2. Determine water level lake (ie. Percent of flooded stations upstream and on lakes increases on upstream lakes and area according to depth of upstream of identify submerged hydraulic inundation). Nonacho Lake controls between lakes 1.2 Outflows from lake. 1. Define explicitly the outflow None 1. None, unless there is poor reliability in the 1. Report daily lake 1999/00 and rating curve of the Nonacho rating curve, and then measure outflows below levels, gate settings, annual Lake control structure. structure for a variety of gate settings inflows, and outflows, reporting for along with expected life of project operating procedures (gate settings) and outflows for one week in advance. 1.3 Traditional and 1. Interview residents and 1. Monitor freeze-up and 1. Survey crossing points during winter to None 1999/00 potential future travel identify historical and potential breakup patterns and the determine bathymetric and hydraulic 2000/01 routes in vicinity of lake future crossing sites and extent/duration of open characteristics. 2001/02 and the ice regime of the frequency of use. water in the vicinity of 2. Measure lake water temperature profiles, snow lake. 2. Identify water level changes potential future or historical depths, and ice thicknesses three times during at these sites and determine crossing sites. each winter at a point upstream of each crossing extent of potential flow changes 2. Identify three to five most site. at the crossing sites critical crossing points for 3. Measure ice thickness, snow depths, and water characterization temperatures in reference lake during each field trip to Nonacho Lake. 1.4 Water temperatures 1. Examine meteorologic data 1. Observe freeze-up and 1. Measure lake water temperature profiles three 1. Provide summaries 1999/00 on Taltson River from stations in vicinity of breakup pattern downstream times during each winter at a point upstream of of daily temperatures 2000/01 downstream of Nonacho basin (Fort Smith, Twin Gorges of control structure control structure. and precipitation for 2001/02 Lake etc.) and determine a method to 2. Measure water temperatures downstream of the basin using provide regional meteorology appropriate annual the control structure at three sites along outlet reporting for for the Taltson River basin. channel on a daily basis meteorologic stations in the region. life of project Table 5.1-1 Monitoring Activities to Address Hydrologic Issues

Activities Reconnaissance Level Reporting/Long Issue Component Preparatory Work Short Term Field Monitoring Schedule Survey Term Monitoring 2. Ice-related 2.1 Freeze up processes 1.Establish a study reach that is 1. Observe the freeze-up 1. Measure the freeze-up levels along a study None 1999/00 water level on Trudel Creek and representative of the overall process five times over a reach in the vicinity of the WSC gauge and relate 2000/01 fluctuations on Taltson River river. period of two to three weeks these levels to the stage variability during freeze- Taltson River downstream of Twin 2. Use the existing surveys to during the freeze-up period, up at the WSC gauge. between Elsie Gorges forebay characterize the channel documenting the following: 2. Measure freeze-up levels at critical use points Falls and Tsu hydraulic characteristics, concentration of flowing ice along the river Lake, particularly differentiating between the formation of ice cover in 3. Measure ice thicknesses over one or two related to freeze- pools and riffles. pools pool/riffle sequences and investigate the up, mid-winter 3. Identify critical areas along upstream progression of ice characteristics of hanging dams if they occur overflows, and the river where ice related cover breakup. flooding and winter crossings open areas within riffles are an issue formation of hanging dams (see 1.2 above) 2.2 Mid-winter ice 1. Compile temperature and 1. Observe the ice 1. Measure water levels at selected critical None 1999/00 conditions and overflow precipitation records for the conditions at monthly locations if ice conditions change dramatically 2000/01 patterns on Trudel Creek area over the winter period. intervals over the winter, over the winter period or if the local ice and Taltson River 2. Contract local observer to noting the following: conditions are not represented by the ice downstream of Twin monitor ice conditions and existence of overflows conditions at the WSC gauge. Gorges forebay water temperatures on a daily aufeis deposits from local 2. Characterize the ice cover as opportunity basis while in area. Photographs inflows presents itself if the water level measurements of ice conditions should be development of open water are being undertaken. made, and local water levels measured. behavior of hanging dams position of the head of the ice cover 2.3 Water temperatures None None 1. Measure water temperatures in Twin Gorges None Annually for on the Taltson River tailrace on a daily basis throughout the year. life of project below Twin Gorges 2. Measure water temperatures on Trudel Creek below South Valley spillway on a weekly basis throughout year. 3. Measure vertical water temperatures in the Twin Gorges forebay upstream of Twin Gorges and South Valley at two week intervals during winter period when ice cover is competent. 4. Measure water temperatures at three locations along Trudel Creek and six locations along the Taltson River between Twin Gorges forebay and Tsu Lake for a period between August and December to establish heat transfer rates at the water surface. Table 5.1-1 Monitoring Activities to Address Hydrologic Issues

Activities Reconnaissance Level Reporting/Long Issue Component Preparatory Work Short Term Field Monitoring Schedule Survey Term Monitoring 5. During late fall, prior to formation of an ice cover, measure the thermal structure of a pool and riffle sequence within the reach defined by the existing channel surveys. 2.4 Breakup process on 1. Identify critical areas along 1. Observe the freeze-up 1. Measure the peak breakup levels along a study Trudel Creek and Taltson the river where ice related process five times over a reach in the vicinity of the WSC gauge and relate River downstream of flooding and winter crossings period of one to two weeks these levels to the stage variability during freeze- Twin Gorges forebay are an issue. during the breakup period, up at the WSC gauge. 2. Establish a study reach that is documenting the following: 2. Measure breakup levels at critical use points representative of the overall thermal erosion of ice front along the river. river. open water development in 3. Measure ice jam profiles and heights of shear riffles walls should ice jams occur. stage increases and ice 4. Measure water temperatures at the head of the cover receding ice cover as opportunities develop effects of hanging dams on during the observations. jams formation of ice dams Table 5.1-1 Monitoring Activities to Address Hydrologic Issues

Activities Reconnaissance Level Reporting/Long Issue Component Preparatory Work Short Term Field Monitoring Schedule Survey Term Monitoring 3. Atypical water 3.1 Outflows from See 1.2 above See 1.2 above See 1.2 above See 1.2 above 1999/00 and level fluctuations Nonacho Lake. then annual below Nonacho reporting for Lake and the life of the downstream of project Twin Gorges, on Trudel Creek, on the Taltson River between Elsie Falls and Tsu Lake, and on the Taltson River between Tsu Lake and the mouth. 3.2 Water level 1. Examine flood routing 1. Identify hydraulic 1. Measure hydraulic characteristics of selected 1. Include the 2001/02 fluctuations at critical procedures for flows along controls along Taltson control(s). expected water level 2002/03 points along waterway Taltson River and identify River, and characterize changes in the weekly between Nonacho Lake attenuation of outflows from according to degree of forecast of water and Twin Gorges Nonacho Lake. control and importance with releases from Forebay 2. Determine water level respect to local water users. Nonacho Lake. fluctuations for selected hydraulic control point(s) that may be due to changes in gate settings at Nonacho Lake, using the data collected in the field. 3.3 Water level 1. Determine rating curve of 1. Examine changes in the 1. If there is poor reliability in the rating curve, 1. Report daily 2000/01 fluctuations along Trudel South Valley spillway water levels in the Twin measure outflows below structure for a variety of outflows, along with 2002/03 then Creek 2. Identify a significant water Gorges forebay during shut gate settings. expected outflows for annual use reach along Trudel Creek down of Twin Gorges. 2. Characterize the slope and hydraulic one week in advance. reporting for 2. Determine routing characteristics of the reach by field surveys 2. Report daily stages the life of the characteristics of flow during summer. along with expected project through the channel and 3. Measure water levels at four different flow stage changes for one lakes along Trudell Creek conditions through the South Valley spillway week in advance for using the response of the the most important WSC gauge below Twin reach along Trudel Gorges to detect changes in Creek flows during shutdown at Twin Gorges. Table 5.1-1 Monitoring Activities to Address Hydrologic Issues

Activities Reconnaissance Level Reporting/Long Issue Component Preparatory Work Short Term Field Monitoring Schedule Survey Term Monitoring 3.4 Water level 1. Identify critical areas where None 1. Measure the hydraulic characteristics of the 1. Report daily 2000/01 fluctuations on the water users may be affected by channel at each of the critical sites (note that the outflows, along with 2001/02 Taltson River below water level changes during the existing surveys on the river can be used if expected outflows for 2003/04 then Twin Gorges open water period. applicable). one week in advance annual 2. Measure water level at four different flows to on a weekly basis. reporting for establish rating curve at site. 2. Report daily stages life of project along with expected stage changes for one week in advance for each of the critical sites. 3.5 Variability in 1. Examine WSC rating curve at None 1. Install gauge at outlet of lake to correlate water 1. Install a permanent 2000/01 outflows from Tsu Lake outlet of lake, determine storage level fluctuations at outlet with those at the hydrometric station 2001/02 due to fluctuations in characteristics of lake, and route existing WSC gauge if analysis indicates that the downstream of Tsu 2002/03 then outflows from Twin typical discharge fluctuations lake does not attenuate short term inflow Lake if analysis annual Gorges/South Valley through lake to determine fluctuations. indicates it is reporting for attenuation effects of lake. warranted life of project 2. Identify critical areas where water users may be affected by water level changes during the open water period.

3.6 Effects of dredging 1. Identify extent and None 1. Survey cross sections at mouth, complete with None 2001/02 on the flow conditions at characteristics of dredging water levels. the mouth of the Taltson program through discussions 2. Calibrate HEC2 model for surveyed River with local authorities conditions, simulate effects of various dredging scenarios on water levels, and assess changes in hydraulic conditions on the freeze-up regime at the mouth Table 5.1-1 Monitoring Activities to Address Hydrologic Issues

Activities Reconnaissance Level Reporting/Long Issue Component Preparatory Work Short Term Field Monitoring Schedule Survey Term Monitoring 4. Sedimentation 4.1 Geomorphic 1. Examine historical flow 1. During summer, describe None None 2000/01 due to erosion and characteristics of the records over the South Valley geomorphic characteristics, 2002/03 instability of the waterway spillway and estimate dominant degree of bedrock control channel platform flows in the waterway. (if any), and locations of Trudel Creek 2. Establish surface area of where channel instability and on the Taltson lakes and determine attenuation may be a problem. River between affects on flows (if any) and Twin Gorges and storage areas for suspended Tsu Lake and the sediment load. Mouth 3. Compare pre and post regulation aerial photography to determine changes in channel platform 4.2 Extent of bank None 1. From above aerial 1. Establish one or two cross sections at one or 1. Measure bank 2002/03 erosion and channel reconnaissance identify two representative erosion sites, and install erosion rates each continuing if instabilities along areas of active erosion reference pins to monitor bank erosion on an year and relate to required waterway annual basis. flows down the 2. Sample the eroding material for grain size waterway. analysis. 4.3 Extent of None 1. Identify candidate sites 1. Establish one or two typical sedimentation None 2002/03 sedimentation along the where sedimentation sites and install sufficient cross sections to continuing if Trudel Creek waterway appears to be occurring. characterize the potential changes in the volume required of the sediment deposit. 2. Sample the deposit for grain size analysis and possibly isotope analysis. 4.4 Suspended sediment None 1. Undertake a visual 1. Measure suspended sediment load at critical None 2002/03 transport along Trudel synoptic survey of areas along the waterway for three or four Creek and the Taltson suspended sediment different flow conditions over the summer River between Twin transport and identify period. Gorges and the Mouth, patterns of erosion and including effects on deposition Deskenatlata Lake. ...J c:::~

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Activities of long-term monitoring, as stated in Component 1.2 (Table 5.1-1), are currently being addressed (daily lake levels and outflows). However, inflows and current gate settings, along with expected gate settings and corresponding outflows have not been addressed. To address these activities of the long-term monitoring program, a robust hydrologic model will be used to compute daily flows and water levels at locations of interest. Model calibration will require short-term flow monitoring. A calibrated and proven model will eliminate the need for continuous long-term flow monitoring at numerous locations within the Taltson River Basin. Long-term flow monitoring will only continue at key locations.

5.1.3 Traditional and Potential Travel Routes

Presently, there is no documentation that explicitly identifies travel routes in the vicinity of ice-covered lakes of the Taltson Hydro Project. Once these travel routes have been identified, the preparatory work, reconnaissance level survey, and short-term field monitoring can be undertaken in the appropriate locations.

5.1.4 Water Temperatures on Taltson River Downstream of Nonacho Lake

Historic water temperature data are presented in Section 4.1 of this report. To address the issues outlined in Table 5.1-1, a robust program that monitors water temperatures in key areas is required. The details of the program are stated in Table 5.1-1.

5.2 Ice-related Water Level Fluctuations

5.2.1 Freeze-up Processes on Trudel Creek downstream of Twin Gorges

Presently, there are no data documenting the freeze-up process on Trudel Creek or on the Taltson River downstream of Twin Gorges. However, long-term flow data are available from both locations. The details of the program required to address these issues are stated in Table 5.1-1.

5.2.2 Mid-winter Ice Conditions and Overflow Patterns

Long-term precipitation and temperature data are available and have been summarized in this report. However, there have been no data collected for mid-winter ice conditions and overflow patterns from Trudel Creek or the Taltson River downstream of Twin Gorges forebay. Once data are obtained, the long-term climate data can be used to estimate historic ice conditions and overflow patterns. The details of the program required to address these issues are stated in Table 5.1-1.

Northwest Territories Power Corporation 5 - 9 Rescan™ Environmental Services Ltd. HYDROLOGICAL ISSUES OF THE WEMP

5.2.3 Water Temperatures on the Taltson River below Twin Gorges

Historic water temperature data are presented in Section 4.1 of this report. To address the issues outlined in Table 5.1-1, a robust program that monitors water temperatures in key areas is required. The details of the program are stated in Table 5.1-1.

5.2.4 Break-up Processes on Trudel Creek and Downstream of Twin Gorges

The identification of critical areas along Trudel Creek and on the Taltson River downstream of Twin Gorges forebay for ice-related flooding and winter crossing will require consultation with the appropriate users. Once these critical areas have been identified, site investigations of these areas, along with field monitoring (as outlined in Component 2.4 of Table 5.1-1) are required.

5.3 Atypical Water Level Fluctuations

5.3.1 Outflows from Nonacho Lake

The outflow pattern from Nonacho Lake (1987-2000) is presented in Section 3.6 of this report. The activities of Component 3.1 (Table 5.1-1) are addressed in Section 5.1.2 of this report.

5.3.2 Water Level Fluctuations Between Nonacho Lake and Twin Gorges Forebay

To characterize water level fluctuations in response to upstream flow regulation, critical hydraulic controls downstream of flow regulation must be identified. The results of a site investigation that identified hydraulic controls through visual inspection are presented in Section 3.2 of this report. These areas require further investigation that detail elevation and dimensions of the hydraulic controls (see Table 5.1-1, Component 3.2).

The existing Taltson Flow Model determines outflows and water levels at numerous lakes along the Taltson River. However, it does not compute results for the critical hydraulic controls. The model can be modified to compute water levels at critical locations once the hydraulic data from these locations have been collected. However, the robustness of the model must be verified through manual flow monitoring.

5.3.3 Water Level Fluctuations Along Trudel Creek

To characterize water levels in a “significant water use reach along Trudel Creek” in response to various discharges from the South Valley Spillway sufficient hydraulic, data will be needed for input into a hydraulic model (e.g., HEC-RAS). The current version of the Taltson Flow Model is not able to compute water level changes on Trudel Creek. The data required to produce a working model of water levels on Trudel Creek are outlined in

Northwest Territories Power Corporation 5 - 10 Rescan™ Environmental Services Ltd. HYDROLOGICAL ISSUES OF THE WEMP

Component 3.3 of Table 5.1-1. Component 3.3 also addresses the need to predict water levels in Trudel Creek a week in advance. This can be done through the use of upstream water level monitoring and predictions of power demands. The area of Trudel Creek to be studied will be determined through discussions with all stakeholders.

5.3.4 Water Level Fluctuations below Twin Gorges

An investigation that addresses water level fluctuations on the Taltson River below Twin Gorges will be similar to that described in Section 5.3.3 (i.e. discussions with stakeholders, and collection of hydraulic and water level data).

5.3.5 Variability of Outflows from Tsu Lake

To address current variability in outflows from Tsu Lake due to fluctuations in outflows from Twin Gorges and the South Valley Spillway, a hydrometric station at the outlet of Tsu Lake is required.

However, sufficient flow data exist from the outlet of Tsu Lake and from Twin Gorges/South Valley Spillway that the attenuation effect of Tsu Lake could be characterized for numerous flow conditions. WSC had a hydrometric station at the outlet of Tsu Lake (07QA001) from 1952 to 1997 (see Section 3.1). Data from this station, along with historic flow data from Twin Gorges Hydro Plant and the South Valley Spillway, can be used to identify the attenuation effects of Tsu Lake. In addition, WSC has operated a station downstream of Twin Gorges Hydro Dam (07QD007) since 1994. Flow comparisons can be made between the two WSC stations to characterize the attenuation capacity of Tsu Lake. The implications of the above investigation to water users will be highlighted.

Before the data from Tsu Lake outflow (07QA001) can be used, the rating curve used to compute flows must be verified through manual flow monitoring.

5.3.6 Effects of Dredging at the Mouth of the Taltson River

To determine the effects of various dredging scenarios at the mouth of the Taltson River cross sections at the mouth along with flow and water level data will be required. Once sufficient data have been collected to calibrate a hydraulic model, simulations with various dredging scenarios can be run. Currently, hydraulic data are not available at the mouth of the Taltson River.

Northwest Territories Power Corporation 5 - 11 Rescan™ Environmental Services Ltd. HYDROLOGICAL ISSUES OF THE WEMP

5.4 Sedimentation Due to Erosion

5.4.1 Geomorphic Characteristics

The geomorphic characteristics of Trudel Creek and Taltson River (between Twin Gorges and Tsu Lake and between Tsu Lake and the Mouth) have not been described. Components of the preparatory work are available and presented in Section 3.7 (flows from the South Valley Spillway) and Section 3.8 (aerial photos of Trudel Creek). However, field investigations will be required to describe the geomorphic characteristics, including: degree of bedrock control, and locations where channel instability may be a problem.

5.4.2 Extent of Bank Erosion and Channel Instabilities Along Waterway

There are no historic data that identify bank erosion and/or channel instability within Trudel Creek and Taltson River between Twin Gorges and Tsu Lake and between Tsu Lake and the Mouth. Reconnaissance work from Component 4.1 (Table 5.1-1) will enable areas of bank erosion and channel instability to be identified so that subsequent field investigations can focus on areas of concern. The details of the program are stated in Table 5.1-1.

5.4.3 Extent of Sedimentation Along Trudel Creek

No background data exist that identify sedimentation concerns along Trudel Creek. Details of the field monitoring required to address sedimentation concerns are stated in Table 5.1.1.

5.4.4 Suspended Sediment Transport Along Trudel Creek and the Taltson River

The work conducted to address Components 4.1 to 4.3 will provide the necessary reconnaissance data necessary to identify patterns of erosion and deposition. The field work required is stated in Component 4.4 (Table 5.1-1).

Northwest Territories Power Corporation 5 - 12 Rescan™ Environmental Services Ltd. REFERENCES

TM REFERENCES

Coyne. 2000. Internal Report: Summary of the Taltson River Basin.

Envirocon Limited, Lee Doran Associates Ltd., and Pearse Bowden Economic Consultants Limited. 1975. Environmental Implications of Proposal to Increase Hydro-Electric Generation on the Taltson River System, Northwest Territories. Prepared for Northern Canada Power Commission. January, 1975.

Envirocon Limited. 1973. Taltson River Hydro Extension Fisheries Study. Interim Data Summary Partial Baseline Inventory. 22 June - 31 August, 1973. Prepared for Northern Canada Power Commission. August, 1973.

Envirocon Limited. 1986. Taltson basin hydroelectric facilities projected impact of Taltson basin hydroelectric facilities on fish populations. Prepared for Northern Canada Power Commission. June, 1986.

Northwest Territories Power Corporation. 1988. Methods for forecasting: i) the Nonacho Dam Releases Required to Maintain Power Generation at the Twin Gorges Plant and: ii) the Effects of these Releases on the Hydraulic Regime of the Downstream Water Courses. Taltson Hydroelectric System (Water Licence #NIL5-0154). December, 1988.

Prasuhn, Alan L. 1938. Fundamentals of Hydraulic Engineering. South Dakota State University, Brookings, South Dakota.

Northwest Territories Power Corporation R - 1 Rescan™ Environmental Services Ltd.