mdu.com under the Investors tab.
Yesterday, we announced third quarter earnings of $139.3 million or $0.68 per share compared to third quarter 2020 earnings of $153.1 million or $0.76 per share.
On a year-to-date basis, we have earned $291.6 million or $1.44 per share compared to the prior year's $277.9 million or $1.39 per share.
Construction Services reported third quarter earnings of $23.1 million compared to the prior year's record third quarter earnings of $29.8 million.
EBITDA at this business decreased $10.9 million from the same period in 2020 to $35.9 million.
Results were negatively impacted by $5.5 million after-tax for changes in estimates on a construction contract during the quarter.
Our Construction Materials business reported earnings of $96.3 million for the third quarter, down from the prior year's $107.3 million.
EBITDA decreased $13.4 million from the same period last year to $158.9 million.
Turning to our Regulated Energy Delivery business, our Combined Utility business reported net income of $5.2 million for the quarter compared to a net loss of $800,000 in the third quarter of 2020.
The Electric Utility segment reported strong third quarter earnings of $20.6 million compared to $16.8 million for the same period in 2020.
Warmer weather helped drive an 11.1% increase in electric retail sales volumes, along with more businesses being open when compared to last year due to pandemic-related impacts.
Our natural gas segment reported an expected seasonal loss of $15.4 million for the quarter, which was a $2.2 million improvement from the previous year.
Higher adjusted gross margin from rate relief and a 2% increase in retail and natural gas sales volumes drove the decreased loss, partially offset by higher O&M expense.
The pipeline business had earnings of $10.6 million in the third quarter compared to $8 million in the third quarter of 2020 primarily from higher AFUDC on the company's North Bakken Expansion project.
Also during the quarter MDU Resources experienced lower income tax benefits of approximately $4.6 million when compared to the third quarter of 2020 related to the timing of recognition of our consolidated annualized estimated tax rate.
On a combined basis, we saw 1.7% customer growth since the same period in 2020 and in the third quarter our natural gas utility refiled in the State of Washington for a $13.7 million annual rate increase that is currently pending.
We expect this fully subscribed project will be in service in early 2022 with capacity to transport 250 million cubic feet of natural gas per day for our customers.
I recently had the opportunity to visit the construction site in North Western North Dakota with other members of our management team and I can tell you first hand, it was an impressive to see over 700 employees and contractors are safely and efficiently working together to complete this $260 million project.
This project involves constructing approximately 60 miles of 12-inch pipeline from our existing facilities at Mapleton, North Dakota, extending to Wahpeton, North Dakota.
It will add 20 million cubic feet per day of natural gas capacity and is expected to cost approximately $75 million.
When the North Bakken and Wahpeton expansion projects are complete, WBI's total system capacity will be more than 2.4 billion cubic feet of natural gas per day, which will help to reduce natural gas flaring in the region and allow producers to move more natural gas to markets.
Construction Services ended the quarter with a backlog of $1.27 billion, down just slightly from the prior year's third year record of $1.28 billion.
We now expect revenues to be in the range of $2.0 billion to $2.2 billion with margins comparable to 2020 levels.
The Knife River training center features and 80,000 square foot heated indoor arena for training on trucks and heavy equipment and an attach 16,000 square foot office with classroom and lab facility.
Revenues are still expected to be in the range of $2.1 billion to $2.3 billion.
Knife River backlog as of September 30 was $651.7 million, a 14% increase from the prior year's $571.3 million.
Based on our results through the third quarter, we have adjusted our earnings per share guidance to now a range of $1.90 to $2.05 per share.

Summary:
mdu.com under the Investors tab.
Yesterday, we announced third quarter earnings of $139.3 million or $0.68 per share compared to third quarter 2020 earnings of $153.1 million or $0.76 per share.
Based on our results through the third quarter, we have adjusted our earnings per share guidance to now a range of $1.90 to $2.05 per share.